OSWER Publication 9200.2-154
OSWER TECHNICAL GUIDE FOR ASSESSING
AND MITIGATING THE VAPOR INTRUSION
PATHWAY FROM SUBSURFACE VAPOR
SOURCES TO INDOOR AIR
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
June 2015
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
DISCLAIMER
This document presents current technical recommendations of the U.S. Environmental
Protection Agency (EPA) based on our current understanding of vapor intrusion into indoor air
from subsurface vapor sources. This guidance document does not impose any requirements or
obligations on the EPA, the states or tribal governments, or the regulated community. Rather,
the sources of authority and requirements for addressing subsurface vapor intrusion are the
relevant statutes and regulations. Decisions regarding a particular situation should be made
based upon statutory and regulatory authority. EPA decision-makers retain the discretion to
adopt or approve approaches on a case-by-case basis that differ from this guidance document,
where appropriate, as long as the administrative record supporting its decision provides an
adequate basis and reasoned explanation for doing so.
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TABLE OF CONTENTS
DISCLAIMER i
TABLE OF CONTENTS ii
LIST OF FIGURES vi
LIST OF TABLES vi
LIST OF APPENDICES vii
ACRONYM SAND ABBREVIATIONS viii
1.0 INTRODUCTION 1
1.1 Definition of Vapor Intrusion 1
1.2 Statutory Authorities 3
1.2.1 Taking Action with Limited Data under CERCLA and the NCR 4
1.2.2 Taking Action with Limited Data under RCRA Corrective Action 6
1.3 Scope and Recommended Uses of this Technical Guide 6
1.3.1 Petroleum Hydrocarbons 9
1.3.2 Nonresidential Buildings 10
1.4 Companion Documents and Technical Resources 10
1.4.1 Vapor Intrusion Screening Level Calculator 11
1.4.2 Technical Support Documents 11
1.5 Historical Context 12
1.6 Public Involvement in Developing Vapor Intrusion Technical Guide 16
1.7 Organization 16
2.0 CONCEPTUAL MODEL OF VAPOR INTRUSION 19
2.1 Subsurface Vapor Sources 22
2.2 Subsurface Vapor Migration 24
2.3 Openings and Driving Forces for Soil Gas Entry into Buildings 26
2.4 Air Exchange and Mixing 30
2.5 Conceptual Model Scenarios 31
2.6 Variability in Exposure Levels 34
2.7 Consideration of Indoor and Outdoor Sources of VOCs 34
3.0 OVERVIEW OF VAPOR INTRUSION TECHNICAL GUIDE 36
3.1 Contaminants of Potential Concern 36
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3.2 Vapor Intrusion Assessment 37
3.3 Building Mitigation and Subsurface Remediation 41
3.4 Community Outreach and Involvement 44
4.0 CONSIDERATIONS FOR NONRESIDENTIAL BUILDINGS 46
5.0 PRELIMINARY ANALYSIS OF VAPOR INTRUSION 49
5.1 Assemble, Evaluate, and Review Available Information 49
5.2 Identify and Respond to Conditions that Warrant Prompt Action 51
5.3 Determine Presence of Structures and Vapor-forming Chemicals 52
5.4 Develop Initial Conceptual Site Model 55
5.5 Evaluating Pre-Existing and Readily Ascertainable Sampling Data 58
5.5.1 Evaluate Sampling Data Reliability and Quality 58
5.5.2 Evaluate Applicability of the VISLs and Adequacy of the Initial CSM 59
5.5.3 Preliminary Risk-based Screening 59
6.0 DETAILED INVESTIGATION OF VAPOR INTRUSION 61
6.1 Common Vapor Intrusion Scenarios 61
6.2 Planning and Scoping 63
6.2.1 Vapor Intrusion Inclusion Zones 67
6.2.2 Prioritizing Investigations with Multiple Buildings 69
6.2.3 Planning for Community Involvement 71
6.3 Characterize the Vapor Intrusion Pathway 71
6.3.1 Characterize Nature and Extent of Subsurface Vapor Sources 72
6.3.2 Characterize Vapor Migration in the Vadose Zone 75
6.3.3 Assess Building Susceptibility to Soil Gas Entry 78
6.3.4 Evaluate Presence and Concentration of Subsurface Contaminants in
Indoor Air 80
6.3.5 Identify and Evaluate Contributions from Indoor and Ambient Air Sources. 81
6.3.6 Select, Prioritize, and Sequence Investigation Objectives 86
6.4 General Principles and Recommendations for Sampling 87
6.4.1 Indoor Air Sampling 88
6.4.2 Outdoor Air Sampling 98
6.4.3 Sub-slab Soil Gas Sampling 99
6.4.4 Soil Gas Sampling 103
6.4.5 Groundwater Sampling 104
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6.4.6 Planningfor Building and Property Access 105
6.5 Overview of Risk-Based Screening 105
6.5.1 Objectives of Screening 105
6.5.2 Scope and Basis for Health-based, Vapor Intrusion Screening Levels 106
6.5.3 Recommended Attenuation Factors for Health-based Screening 108
6.5.4 Comparing Sample Concentrations to Health-based Screening Levels.... 111
6.5.5 Planning for Communication of Sampling Results 112
6.6 General Principles and Recommendations for Mathematical Modeling 113
7.0 RISK ASSESSMENT AND MANAGEMENT FRAMEWORK 117
7.1 Collect Site-specific Lines of Evidence 117
7.2 Weigh and Assess Concordance Among the Lines of Evidence 120
7.3 Evaluate Whether the Vapor Intrusion Pathway is Complete or Incomplete 122
7.4 Conduct and Interpret Human Health Risk Assessment 124
7.4.1 Risk Management Benchmarks 127
7.4.2 Accounting for Background Contributions 128
7.4.3 Occupational Exposure Limits 128
7.5 Concentration Levels Indicating Potential Need for Prompt Response Action 129
7.5.1 Potential Explosion Hazards 129
7.5.2 Considering Short-term and Acute Exposures 129
7.6 Risk-based Cleanup Levels 131
7.7 Options for Response Action 132
7.8 Pre-emptive Mitigation/Early Action 134
7.8.1 Rationale 135
7.8.2 General Decision Framework 136
7.8.3 Some General Scenarios Where Pre-emptive Mitigation May be
Warranted 137
7.8.4 Additional Considerations 141
8.0 BUILDING MITIGATION AND SUBSURFACE REMEDIATION 143
8.1 Subsurface Remediation for Vapor Source Control 143
8.2 Building Mitigation for Vapor Intrusion 144
8.2.1 Prompt Response Options for Existing Buildings 144
8.2.2 Active Depressurization Technologies for Existing Buildings 147
8.2.3 Approaches and Considerations for New Buildings 150
8.2.4 Owner/Occupant Preferences and Building Access 151
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8.3 Operation and Maintenance of Vapor Intrusion Mitigation Systems 151
8.4 Monitoring of Vapor Intrusion Mitigation Systems 153
8.5 Documentation of Engineered Exposure Controls for Vapor Intrusion Mitigation... 156
8.6 Use of Institutional Controls 157
8.6.1 Evaluating ICs in the Overall Context of Response Selection 158
8.6.2 Common Considerations and Scenarios Involving ICs 159
8.6.3 Selectingthe Right Instrument(s) 163
8.6.4 Long-term Stewardship 165
8.6.5 Community Involvement and ICs 166
8.7 Termination/Exit Strategy 167
8.7.1 Termination of Subsurface Remediation Activities 167
8.7.2 Termination of Building Mitigation 168
8.7.3 Termination of ICs 170
8.7.4 Termination of Monitoring 170
9.0 PLANNING FOR COMMUNITY INVOLVEMENT. 175
9.1 Developing a Community Involvement or Public Participation Plan 177
9.2 Communication Strategies and Conducting Community Outreach 179
9.3 Addressing Building Access for Sampling and Mitigation 182
9.4 Communication of Indoor Sampling Efforts and Results 183
9.5 Transmitting Messages Regarding Mitigation Systems 185
9.6 Addressing Community Involvement at Legacy Sites 186
9.7 Property Value Concerns for Current and Prospective Property Owners 187
9.8 Additional Community Involvement Resources 187
10.0 GLOSSARY 189
11.0 CITATIONS AND REFERENCES 202
APPENDIX A Recommended Subsurface-to-lndoor Air Attenuation Factors A-1
APPENDIX B Data Quality Assurance Considerations B-1
APPENDIX C Calculating Vapor Source Concentration from Groundwater Data C-1
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LIST OF FIGURES
Figure 2-1 Illustration of Key Elements of the Conceptual Model of Soil Vapor Intrusion
Figure 2-2 Illustration of Potential Openings in Various Building Types
Figure 2-3 Some Factors That Affect Vapor Intrusion
Figure 3-1 Overview of Recommended Framework for Vapor Intrusion Assessment and
Response Action
Figure 6-1 Overview of Planning, Scoping, and Conducting Vapor Intrusion Investigations
Figure 7-1 Sample Depiction of Subsurface Vapor Source and Data to Support Pre-emptive
Mitigation/Early Action for Multiple Buildings, Each with Limited Data
Figure 7-2 Sample Depiction of Subsurface Vapor Source and Data to Support Pre-emptive
Mitigation/Early Action for Multiple Buildings, Some with Only Limited or No Data
Figure 8-1 Illustration of Sub-slab Depressurization (SSD) System
Figure 8-2 Illustration of Drain-tile Depressurization (DTD) System
Figure 8-3 Illustration of Block-wall Depressurization (BWD) System
Figure 8-4 Illustration of Sub-membrane Depressurization (SMD) System
LIST OF TABLES
Table 1 -1 Directory to Updates in This Technical Guide Addressing Recommendations of
EPA Office of Inspector General (EPA 2009)
Table 1 -2 Directory to Additional Updates in This Technical Guide Publicly Identified by
OSWER(EPA2010A)
Table 1 -3 Vapor Intrusion Topics Receiving Substantive Public Comment
Table 6-1 Recommended Vapor Attenuation Factors for Risk-based Screening of the Vapor
Intrusion Pathway
Table 7-1 Matrix of Options to Respond to Human Health Risk Posed by the Vapor
Intrusion Pathway
Table 8-1 Vapor Intrusion Mitigation Quick Guide for Existing Buildings
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LIST OF APPENDICES
Appendix A Recommended Subsurface-to-lndoor Air Attenuation Factors
Appendix B Data Quality Assurance Considerations
AppendixC Calculating Vapor Source Concentration from Groundwater Sampling Data
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ACRONYMS AND ABBREVIATIONS
ACH air changes per hour (air exchanges per hour)
ADT active depressurization technology
AER air exchange rate
ANSI American National Standards Institute
ASQ American Society for Quality
ASTM American Society for Testing and Materials
ASTSWMO Association of State and Territorial Solid Waste Management Officials
ATSDR Agency for Toxic Substances and Disease Registry
BTEX benzene, toluene, ethylbenzene, xylenes
BWD block-wall depressurization
CalEPA California Environmental Protection Agency
CASRN Chemical Abstracts Service Registry Number
CEI Community Engagement Initiative
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act
CFR Code of Federal Regulations
CMC chlorinated hydrocarbon
CIC Community Involvement Coordinator
CIO Chief Information Officer
CIP community involvement plan
CMS corrective measures study
CSM conceptual site model
DCE dichloroethylene, orequivalentlydichloroethene
DNAPL dense non-aqueous-phase liquid
DoD U.S. Department of Defense
DoN U.S. Department of Navy
DQO data quality objective
DTD drain-tile depressurization
El environmental indicator
EPA U.S. Environmental Protection Agency
ERT Environmental Response Team
FR Federal Register
FS feasibility study
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FYR five-year review
HI Hazard Index
HQ Hazard Quotient
HVAC heating, ventilation and air conditioning
1C institutional control
ICIAP Institutional Controls Implementation and Assurance Plan
IDLH immediately dangerous to life or health
ITRC Interstate Technology and Regulatory Council
LCR lifetime cancer risk
LEL lower explosive limit
LEP limited English proficiency
LNAPL light non-aqueous-phase liquid
LTS long-term stewardship
MADEP Massachusetts Department of Environmental Protection
NAPL non-aqueous-phase liquid
NAS National Academy of Sciences
NCP National Oil and Hazardous Substances Pollution Contingency Plan
NFA No Further Action
NIST National Institute of Standards and Technology
NPL National Priorities List
NRC National Research Council
NIOSH National Institute for Occupational Safety and Health
NYSDOH New York State Department of Health
O&M operation and maintenance
OIG Office of the Inspector General
OSC on-scene coordinator
OSHA Occupational Safety and Health Administration
OSWER Office of Solid Waste and Emergency Response
OUST Office of Underground Storage Tanks
PAH polycyclic aromatic hydrocarbon
PCB polychlorinatedbiphenyl
PCE tetrachloroethylene, orequivalently tetrachloroethene
PEM preemptive mitigation
PID photoionization detector
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P.E. Professional Engineer
ppbv parts per billion by volume
PRP potentially responsible party
QAPP quality assurance project plan
QMP quality management plan
RCRA Resource Conservation and Recovery Act
RfC inhalation reference concentration
RFI RCRA facility investigation
Rl remedial investigation
RME reasonable maximum exposure
ROD Record of Decision
RPM remedial project manager
SMD sub-membrane depressurization
SSD sub-slab depressurization
TAGA trace atmospheric gas analyzer
TCE trichloroethylene, orequivalentlytrichloroethene
UFP-QAPP Uniform Federal Policy for Quality Assurance Project Plans
UECA Uniform Environmental CovenantsAct
USPS U.S. Postal Service
UST underground storage tank
UU/UE unlimited use/unrestricted exposure
VI vapor intrusion
VISL vapor intrusion screening level
VOC volatile organic compound
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Vapor intrusion is the general term given to migration of hazardous vapors from any subsurface
vapor source, such as contaminated soil or groundwater, through the soil and into an overlying
building or structure. These vapors can enter buildings through cracks in ' basements and
foundations, as well as through conduits and other openings in the building envelope. Vapors
can also enter structures that are not intended forhuman occupancy (e.g., sewers, drain lines,
access vaults, storage sheds, pump houses) through cracks and otheropenings.
All types of buildings, regardless of foundation type (e.g., basement, crawl space, slab-on-
grade), have openings that renderthem potentially vulnerable to vapor intrusion. Buildings
subject to vapor intrusion include, but are not limited to, residential buildings (e.g., detached
single-family homes, trailer or'mobile' homes, multi-unit apartments and condominiums),
commercial workplaces (e.g., office buildings, retail establishments), educational and
recreational buildings (e.g., schools and gyms), and industrial facilities (e.g., manufacturing
plants).
Vapor intrusion is a potential human exposure pathway - a way that people may come into
contact with hazardous vapors while performing their day-to-day indoor activities. For purposes
of this Technical Guide, the vapor intrusion pathway is referred to as "complete" fora specific
building or collection of buildings when the following five conditions are met undercurrent
conditions:
1) A subsurface source of vapor-forming chemicals is present (e.g., in the soil or in
groundwater) underneath or near the building(s) (see Sections 2.1, 5.3, 6.2.1, and
6.3.1);
2) Vapors form and have a route along which to migrate (be transported) toward the
building (see Sections 2.2 and 6.3.2);
3) The building(s) is(are) susceptible to soil gas entry, which means openings exist for
the vapors to enter the building and driving 'forces' (e.g., air pressure differences
between the building and the subsurface environment) exist to drawthe vapors from
the subsurface through the openings into the building(s) (see Sections 2.3 and
6.3.3);
4) One or more vapor-forming chemicals comprising the subsurface vaporsource(s)
is(are) present in the indoorenvironment (see Sections 6.3.4 and 6.4.1); and
5) The building(s)1 is(are) occupied by one or more individuals when the vapor-forming
chemical(s) is(are) present indoors.
1 For purposes of this Technical Guide and its recommendations for evaluating human health risk posed by vapor-
forming chemicals, "building" refers to a structure that is intended for occupancy and use by humans. This would
include, forinstance, homes, offices, stores, commercial and industrial buildings, etc., butwould not normally include
sheds, carports, pump houses, or other structures that are not intended forhuman occupancy.
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A complete vapor intrusion pathway indicates that there is an opportunity for human exposure,
which warrants further analysis (see Section 7.4) to determine whether there is a basis for
undertaking a response action(s) (see Section 7.7). Depending upon building- and site-specific
circumstances, concentrations of chemical vapors indoors arising from a complete vapor
intrusion pathway may threaten the health of building occupants (e.g., residents, workers, etc.),
which may warrant a response action(s).
On the other hand and for purposes of this Technical Guide, if one (or more) of the five
foregoing conditions is currently absent and is reasonably expected to be absent in the future
(e.g., vapor migration is significantly and persistently impeded by natural geologic, hydrologic, or
biochemical (e.g., biodegradation) processes and conditions), the vapor intrusion pathway is
referred to as "incomplete." EPA recommends that any determination that the vaporintrusion
pathway is incomplete be supported by site-specific evidence to demonstrate that the nature
and extent of vapor-forming chemical contamination in the subsurface has been well
characterized (Section 6.3.1) and the types of vapor sources and the conditions of the vadose
zone and surrounding infrastructure do not present opportunities for unattenuated or enhanced
transport of vapors (Sections 5.4 and 6.5.2) toward and into any building (see Section 7.3 for
further discussion). When the vapor intrusion pathway is determined to be incomplete, then
vapor intrusion mitigation is not generally warranted.
EPA recommends that site managers also evaluate whethersubsurface vapor sources that
remain have the potential to pose unacceptable human health risks due to vaporintrusion in the
future2 if site conditions were to change. The vapor intrusion pathway is referred to as
'potentially complete' for a building when:
a subsurface source of vapor-forming chemicals is present underneath or near an
existing building or a building that is reasonably expected to be constructed in the future;
vapors can form from this source(s) and have a route along which to migrate (be
transported) toward the building; and
three additional conditions are reasonably expected to all be met in the future, which
may not all be met currently; i.e.,
o the building is susceptible to soil gas entry, which means openings exist for the
vapors to enter the building and driving forces exist to drawthe vapors from the
subsurface through the openings into the building;
o one or more vapor-forming chemicals comprising the subsurface vaporsource(s)
is (or will be) present in the indoor environment; and
o the building is or will be occupied by one or more individuals when the vapor-
forming chemical(s) is (or are) present indoors.
"Both current and reasonably likely future risks need to be considered in order to demonstrate that a site does not
present an unacceptable risk to human health and the environment." (EPA 1991a).
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In addition to their toxicity threats, methane and certain other vapor-forming chemicals can also
pose explosion hazards depending upon structure-, building-, and site-specific circumstances.
Explosion hazards may pose an imminent and substantial dangerto human health and public
welfare.
To help assess the subsurface vapor intrusion pathway, the Office of Solid Waste and
Emergency Response (OSWER) released in November 2002 for comment EPA's Draft
Guidance for Evaluating the Vapor Intrusion to Indoor Air Path way from Groundwater and Soils
("Draft VI Guidance"). Since the Draft VI Guidance was released, EPA's knowledge of and
experience with assessment and mitigation of the vapor intrusion pathway has increased
considerably, leading to an improved understanding of and enhanced approaches for evaluating
and managing vapor intrusion. In addition, EPA received hundreds of comments from the public
since 2002 on the Draft VI Guidance, on a public review draft issued in April 2013, and on
emerging practices and science considerations.
This Technical Guide presents current technical recommendations of the EPA based on our
current understanding of vapor intrusion into indoorair from subsurface vapor sources. One of
its main purposes is to promote national consistency in assessing the vapor intrusion pathway.3
At the same time, it provides a flexible science-based approach to assessment that
accommodates the different circumstances (e.g., stage of the cleanup process) in which vapor
intrusion is first considered at a site and differences among pertinent EPA programs. This
Technical Guide is intended for use at any site (and any building or structure on a site) being
evaluated by EPA pursuant to the Comprehensive Environmental Response, Compensation,
and Liability Act (CERCLA) or the corrective action provisions of the Resource Conservation
and Recovery Act (RCRA), EPA's brownfield grantees, or state agencies acting pursuant to
CERCLA or an authorized RCRA corrective action program where vapor intrusion may be of
potential concern. This document and the accompanying Technical Guide For Addressing
Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites (EPA 2015b)4
supersede and replace the Draft VI Guidance.
Although this Technical Guide is intended for use at any site subject to federal statutes,
regulations, and rules, it is not intended to alter existing requirements, guidance, or practices
among OSWER's programs about development, selection, or documentation of final
remediation5 plans (addressing subsurface vaporsources, for example).
3 If EPA staff wish to consider using any specific guidance that is not explicitly recommended in this Technical Guide,
they should consult with Headquarters.
4 For petroleum hydrocarbons that arise from petroleum that has been released from Subtitle I LIST systems, EPA
has developed a companion to this Technical Guide (Technical Guide For Addressing Petroleum Vapor Intrusion At
Leaking Underground Storage Tank Sites (EPA 2015b)), which provides information and guidance about how EPA
recommends vapor intrusion be assessed for petroleum hydrocarbons in these settings.
5 For purposes of this Technical Guide, "remediation" is intended to apply to interim and final cleanups, whether
conducted pursuant to RCRA corrective action, the CERCLA removal or remedial programs, or using EPA brownfield
grant funds with oversight by state and tribal response programs. In addition to permanent remedies for subsurface
vaporsources, site remediation may also entail implementation of institutional controls and construction and
operation of engineered systems for exposure control.
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This document is comprised of eleven sections and three appendices, including a list of
acronyms, which precedes this summary, and a glossary of terms in Section 10. Section 3
provides an overview of the entire Technical Guide and can be furthersummarized as follows:
Broadly speaking, two general levels of vapor intrusion assessments can be
distinguished:
1) A preliminary analysis, which utilizes available and readily ascertainable information
to develop an initial understanding of the potential for human health risks to be posed
by vapor intrusion, which would typically be performed as part of an initial site
assessment (Section 5).
2) A detailed investigation (Section 6), which is generally recommended when the
preliminary analysis (e.g., Section 5.3) indicates that subsurface contamination with
vapor-forming chemicals may be present underlying or near buildings. A detailed
investigation of the vapor intrusion pathway is typically performed as part of the site
investigation stage.
The approach for assessing vapor intrusion will vary from site to site, because each site
will differ in the available data when vapor intrusion is being evaluated. This Technical
Guide, therefore, recommends a framework for planning and conducting vapor intrusion
investigations, rather than a prescriptive step-by-step approach to be applied at every
site.
Response actions to address vapor intrusion when it poses unacceptable human health
risks (Sections 7.7 and 8) typically entail a combination of:
o remediation to reduce or eliminate subsurface vaporsources (Section 8.1);
o engineered exposure controls for specific buildings to reduce vapor intrusion or
reduce concentrations of vapor-forming chemicals that have already entered the
building (Section 8.2);
o monitoring to assess and verify the performance and effectiveness of the
remediation systems and engineered exposure controls (Section 8.4); and
o institutional controls (ICs) to restrict land use and/orto alert parties (e.g.,
prospective developers, owners, and municipalities) of the presence of
subsurface sources of vapor-forming chemicals and to foster operation,
maintenance, and monitoring of the remediation systems and engineered
exposure controls (Section 8.6).
Additional response actions to avoid or reduce human exposure may also warrant
consideration in circumstances where "early or prompt response action is appropriate to
address indoor air exposure conditions or a potential for explosion hazards.
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Additional key recommendations and policies comprising this document include the following:
Planning, Scoping and Conducting Investigations (see Sections 5.4 and 6.2 for further
information)
Consider site and building access agreements, equipment security, and locations of
underground utilities and piping, such as storm and sewer lines within buildings, when
planning vapor intrusion investigations (Section 6.2).
Develop an initial conceptual site model, use this model to guide planning and scoping of
the investigation, and update this model as additional information and insights are
generated (Sections 5.4, 7.1, and 7.2).
Generally limit chemical analyses to those vapor-forming chemicals known or
reasonably expected to be present in the subsurface environment.
Consider a "worst first" approach to prioritize buildings for investigation at sites where
numerous buildings are potentially subject to vapor intrusion (Section 6.2.2).
To the extent practical, plan and implement investigations within buildings and on
individual properties with the goal of limiting return visits, which can cause disruption and
inconvenience for building occupants and owners (Section 6.2).
Generally assess the vapor intrusion pathway by collecting, weighing, and evaluating
multiple lines of evidence (Sections 6.3, 7.1, and 7.2).
Utilize 100 feet to define an initial lateral inclusion zone for vapor intrusion assessment
(i.e., for identifying buildingsthat are 'near" a subsurface vaporsource and generally
warrant assessment) for purposes of a preliminary analysis). Investigate soil vapor
migration distance (e.g., define inclusion zone(s) for assessing vapor intrusion in specific
buildings) on a site-specific basis. That is, distances larger or smaller than 100 feet (i.e.,
beyond or within an initial 100-foot inclusion zone) may need to be considered when
developing objectivesfor detailed vapor intrusion investigations and interpreting the
resulting data (Section 6.2.1).
To support evaluations of sources of indoor air concentrations, identify in individual
buildings known or suspected indoor sources of the vapor-forming chemicals also found
in the subsurface and characterize ambient air quality in the site vicinity for these same
chemicals (Sections 6.3.5 and 6.4).
Select sampling and analytical methods that are capable of obtaining reliable analytical
detections of concentrations less than project-appropriate, risk-based screening levels
(e.g., vapor intrusion screening levels, orVISLs).
When groundwater is a subsurface source of vapors, collect groundwater samples from
wells screened across the top of the water table to characterize the source strength for
vapor intrusion (Sections 6.3.1 and 6.4.5).
Collect indoor air samples to characterize exposure levels in indoorair, account for
seasonal variations in climate and the habits of building occupants, and ensure that
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related risk management decisions are based upon a consideration of a reasonable
maximum vapor intrusion condition for a given building (Sections 6.3.4 and 6.4.1).
When sampling indoor air,
o employ time-integrated sampling methods (e.g., evacuated canisters, sorbent-based
sampling devices). Indoor air concentrations can be temporally variable and time-
integrated exposure estimates over appropriate exposure durations (e.g., chronic
typically; less-than-chronic in some cases) are generally most useful for exposure
and human health risk assessment (Sections 6.4.1 and 7.4);
o remove potential indoor sources6 of vapor-forming chemicals from the building to
strive to ensure that the concentrations measured in the indoorair samples are
attributable to the vaporintrusion pathway (Sections6.3.5 and 6.4.1); and
o measure the pressure difference between the indoors and the subsurface, which
provides a complementary line of evidence to support data evaluation and
interpretation (Section 6.4.1) and is a more direct means of assessing building
under-pressurizationthan is monitoring weather/climate factors (e.g., air
temperature, wind speed).
Mathematical modeling of vapor intrusion is most appropriately used in conjunction with
other lines of evidence (Section 6.6).
Confirm the reliability of modeling results, especially when limited site-specific data are
available as inputs (Section 6.6).
Collect and evaluate appropriate site-specific information to demonstrate that the
property fulfills the conditions and assumptions of the generic conceptual model
underlying the vapor intrusion screening levels (Section 6.5.2).
Data Evaluation and Decision-making
Assess (and seek) concordance among the lines of evidence to more confidently
support decision-making (Sections 7.1, 7.2, and 7.3).7 Multiple lines of evidence are
generally recommended forsupporting conclusions, such as the following:
o The subsurface vapor source(s) at a specific site has the potential to pose an
unacceptable vapor intrusion exposure undercurrent or reasonably expected future
conditions, due to its vapor strength (Section 6.5) and proximity relative to one or
more existing buildings or a building that may be constructed in the future (Section
6.2.1).
As mentioned in Section 6.3.5, indoor sources can sometimes be identified and located using portable instruments.
7 Confidence in the assessment and risk management decisions is expected to be higher when multiple independent
lines of evidence come together to provide mutually supporting evidence for a common understanding of the site
conditions/scenarios and the potential for vapor intrusion (EPA 2010b).
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o The vapor intrusion pathway is complete for one or more buildings under current or
reasonably expected future conditions (Section 7.3).
o The vapor intrusion pathway is incomplete for one or more buildings near a
subsurface source of vapor-forming chemicals (Section 7.3), due to
inadequate source strength (i.e., chemicals comprising subsurface
contamination and/ortheir potential vapor concentrations cannot pose an
unacceptable human health risk via the vapor intrusion pathway) (Section
6.5); or
geologic, hydrologic, and/orbiochemical (e.g., biodegradation) processes that
provide substantial and persistent attenuation of vapors extending laterally
over large distances relative to the footprint of the building(s) and the extent
of the vapor source (Section 6.3.2).
o Indoor air concentrations attributable to vapor intrusion pose (or, alternatively, are
unlikely to pose) an unacceptable human health risk in one or more existing buildings
under current or reasonably expected future conditions, based upon currently
available information abouta chemical's toxicity (Section 7.4).
o Indoor air concentrations measured in one or more buildings can (or alternatively,
cannot) be reasonably attributed to indoor or ambient air sources (i.e., background -
see Glossary) (Sections 6.3.5 and 7.4.2).
Multiple lines of evidence are particularly important for supporting "no-further-action"
decisions regarding the vapor intrusion pathway (e.g., pathway incomplete
determinations) to reduce the chance of reaching a false-negative conclusion (i.e.,
concluding vapor intrusion does not pose unacceptable human health risk, when it
actually poses an unacceptable human health risk). Collecting and weighing multiple
lines of evidence can also reduce the chance of reaching a false-positive conclusion
(i.e., concluding vapor intrusion poses unacceptable human health risk, when it does
not). On the other hand, parties may implement engineered exposure controls for vapor
intrusion, even though only limited lines of evidence or measurements may be available
to characterize the overall vapor intrusion pathway.
Consider reasonably expected future conditions, in addition to current conditions, when
reaching conclusions about the vapor intrusion pathway (Sections 3.2 and 7.3). For
example, EPA recommends that vapor intrusion be evaluated for reasonably expected
future land use conditions, including new building construction and new uses and
occupants for uninhabited buildings.
Identify any conditions that warrant prompt action (Section 7.5) and respond, consistent
with applicable statutes and regulations and considering EPA guidance, with actions that
eliminate, avoid, reduce or otherwise address the human health risk posed by vapor
intrusion (Sections 7.7 and 8.2):
o Explosive conditions and threats that warrant prompt action (Section 7.5.1) are
reasonably suspected to exist when measured concentrations of vapors in the
building, utility conduits, sumps, subsurface drains, or other structure directly
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connected to the building exceed one-tenth (10%) of the lower explosive limit (LEL).8
EPA recommends evacuation of buildings with potential explosion and fire hazards,
along with notification to the local fire department about the situation.
o Conditions posing health concerns that warrant prompt action are reasonably
suspected to exist when estimated exposure concentrations of vapors in the building
exceed health-protective concentrations for short-term or acute exposure (Section
7.5.2).
When making decisions pertaining to the assessment of vapor intrusion atnonresidential
buildings, consider the characteristics of the populations potentially exposed to vapor-
forming chemicals in the indoor air, the relative contributions of vapors from background
(including anthropogenic background), and any existing or planned engineering or
institutional controls for the building, in addition to the potential for vapor intrusion
(Sections 4, 6.3.5, 6.4.1 and 7.4.2).
When evaluating environmental sampling results to assess the vapor intrusion pathway,
first determine that the samples were collected appropriately (Sections 5.5 and 6.4).
Before conducting risk-based screening, verify that the site fulfills the conditions and
assumptions of the generic conceptual model underlying the VISLs (Section 6.5.2).
Compare groundwater concentrations to the VISLs (Section 6.5) for groundwater to
estimate the boundaries of the plume, when contaminated groundwater is the
subsurface vaporsource forvapor intrusion (Section 6.2.1).
Generally support the decision to collect indoor air data (Section 6.4.1) by lines of site-
or building-specific evidence that demonstrate vapor intrusion has the potential to pose a
significant human exposure [e.g., data on strength and proximity of subsurface vapor
source(s) (Sections6.2.1, 6.3.1, and 6.5), or preferential vapor migration in the vadose
zone or into buildings (Sections 5.4, 6.3.2, and 6.3.3)].
Consider variability in laboratory analyses when evaluating sampling data.
Generally conduct a human health risk assessment to determine whether the potential
human health risks posed to building occupants are within or exceed acceptable levels
consistent with applicable statutes and considering EPA guidance (Section 7.4).
Consider the potential for adverse non-cancer health effects from short-duration
exposures (i.e., acute, short-term, or subchronic exposure durations), as well as longer
term exposure (i.e., chronic exposure) conditions, and select toxicity values considering
OSWER's preferred hierarchy of sources (EPA 2003) (Sections 7.4 and 7.5.2).
The Occupational Safety and Health Administration of the U.S. Department of Labor (OSHA) considers
concentrations in excess of one-tenth of the LEL to be a hazardous atmosphere in confined spaces [29 CFR
1910.146(b)]. The National Institute for Occupational Safety and Health (NIOSH) has designated such concentrations
as immediately dangerous to life or health (IDLH).
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Consider collecting multiple rounds of indoor air samples,9 using time-integrated
measurements (Section 6.4.1) to estimate exposure concentrations appropriate for the
exposure (occupancy) scenario being evaluated (e.g., residential versus commercial),
when the risk assessment for an existing building would support a conclusion that the
human health risks are acceptable (Section 7.4).10
In the risk characterization of the human health risk assessment, discuss 'background'
contributions to indoor air exposure and associated human health risks (Section 7.4.2).
(For purposes of this Guide, 'background' refers to a vapor-forming chemical(s) or
location(s) that is(are) not influenced by the releasesfrom a site - see Glossary).
Information on'background' contributions of site-related, vapor-forming chemicals in
indoor air is important to risk managers because generally EPA does not clean up to
concentrations below natural or anthropogenic background levels11 (EPA2002e).
If data are available, distinguish the contribution of 'background' to total exposure
concentration(s). With such information, EPA can help advise affected individuals about
the environmental and human health risks they face. Other parties, including building
owners and operators, may help with risk communication.
If background vapor sources (see Glossary) are found to be primarily responsible for
indoor air concentrations (see Section 6.3.5), then response actions for vapor intrusion
would generally not be warranted for current conditions.
Engineered Exposure Controls and Building Mitigation
When vapor intrusion has been determined to pose unacceptable human health risks,
Aim to achieve a permanent remedy by eliminating or substantially reducing the
level(s) of vapor-forming chemical(s) in the subsurface source medium (e.g.,
groundwater, subsurface soil, sewer lines) (Sections 7.7 and 8.1); and
o In cases where subsurface vapor sources cannot be remediated quickly, implement
engineered exposure controls to reduce or eliminate vaporintrusion in buildings (i.e.,
"mitigate" vapor intrusion) or reduce indoor air exposure levels (Sections 7.7 and
8.2).
9 Because weather conditions and building operations can lead to time-variable contributions from vapor intrusion and
ambient air infiltration, indoor air concentrations of vapor-forming chemicals can be expected to vary over time (see,
for example, Section 2.6). An individual sample (orsingle round of sampling) would be insufficient to characterize
seasonal variability, or variability at any other time scale.
EPA recommends basing the decision about whetherto undertake response action for vapor intrusion (i.e., a
component of risk management; see Section 7.4) on a consideration of a reasonable maximum exposure (e.g., EPA
1989, 1991a), w hich is a semi-quantitative term, referring to the lower portion of the high end of the exposure
distribution (see Glossary).
11 With respect to vapor intrusion mitigation (see Sections 3.5 and 8.2), some options for reducing indoor air exposure
levels (e.g., ventilation, indoor air treatment) unavoidably acton background concentrations arising from indoor or
outdoor sources, as well as vapor concentrations arising from vapor intrusion. Most options for interrupting the vapor
intrusion pathway (e.g., active depressurization technologies -see Section 8.2) unavoidably interrupt the intrusion of
naturally occurring radon also. It should also be noted that some EPA regulations (e.g., indoor radon standards under
40 CFR 192.12) are inclusive of background.
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When developing monitoring programs to assess effectiveness of building mitigation,
consider the degree of human health risk or hazard being mitigated, the building use, the
technology used to mitigate vapor intrusion, and coordination with site remediation
efforts.
Establish cleanup levels and criteria forterminating engineered exposure controls and
other building mitigation methods, institutional controls, and remediation systems for
subsurface vaporsources (Sections 7.6 and 8.7).
Document Activities and Decisions
Document objectives and methods of vapor intrusion investigations, preferably in a
vapor intrusion work plan (Section 6.2).
Base decisions upon data and information in the administrative record.
Base decisions to undertake response actions on lines of site- or building-specific
evidence (e.g., characterization of subsurface vapor source(s) strength and proximity to
building(s); building conditions) that demonstrate that vapor intrusion has the potential to
pose an unacceptable human health risk (Section 7).
Document, consistent with statutory requirements and considering prevailing guidance
for the respective land restoration program (e.g., CERCLA, RCRA corrective action,
brownfields, etc.), any and all decisions pertaining to vapor intrusion, including decisions
to undertake (or not to undertake) investigation or mitigation of specific buildings at a
contaminated site.
Prepare and publish system manuals to document building mitigation and remediation
systems (Section 8.5).
Prepare and implement operations and maintenance manuals and practices to foster
continued effective operation and performance of engineered exposure controls and
remediation systems for subsurface vaporsources (Section 8.5).
Document monitoring programs that assess the performance and effectiveness of
remediation and mitigation systems.
Community Outreach and Involvement (Section 9)
Develop or refine a community involvement or public participation plan while planning a
vapor intrusion investigation and implement this plan throughout the assessment,
remediation, and mitigation phases.
Conduct building-by-building contact and communication as means of educating the
community and obtaining access needed to assess, mitigate, and monitor the vapor
intrusion pathway. Personal contact is further recommended to establish a good working
relationship with each building owner or occupant and to build trust.
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Generally provide validated results and interpretations (e.g., chemicals of concern,
associated risk assessment implications) to property owners and occupants in a timely
manner (e.g., within approximately 30 days of receiving these results).
Provide adequate opportunities for public participation (including potentially affected
landowners and communities) when considering appropriate use of ICs.
The science and technology to assess and mitigate vapor intrusion have evolved significantly
over the past decade. EPA will continue to monitor these evolving developments and will update
these recommendations in the future, if and as appropriate.
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1.0 INTRODUCTION
This technical guide was prepared by the U.S. Environmental Protection Agency (EPA) through
the cooperative efforts of a team of EPA Headquarters and Regional staff, known as the Vapor
Intrusion Intra-Agency Workgroup (Workgroup). Drafts of this document were subjected to a
comprehensive, consultative peer-input process in 2012, as described in EPAs Peer Review
Handbook (EPA-SPC 2006), which included comments and othercontributions from Workgroup
members representing several EPA offices and the EPAs Vapor Intrusion Forum.12 Public
comments submitted from 2002 through 2013 and recommendations of the Office of Inspector
General (OIG) were also considered in developing this document.
This document comprises EPAs 'final' vapor intrusion technical guide13 and is referred to herein
as "this Technical Guide." It describes a recommended framework for assessing vapor intrusion
that relies upon collecting and evaluating multiple lines of evidence to support risk management
decisions. It also provides technical recommendations about monitoring and terminating building
mitigation systems.
This Technical Guide relied upon a large body of scientific information found in the peer-
reviewed literature. Additionally, EPA developed three technical support documents that were
externally peer reviewed (EPA 2011 a, 2012a, 2012b). This approach is consistent with EPAs
peer review handbook and policy (EPA-SPC 2006). Peer-reviewed literature, peer-reviewed
technical reports, existing and relevant EPA guidance (e.g., for conducting human health risk
assessment; for planning and conducting investigations of environmental contamination), and
other pertinent information that support development or implementation of this Technical Guide
are cited within.
This introductory section: defines the term "vapor intrusion"; summarizes EPAs statutory
authorities to protect human health from vapor intrusion; summarizes the intended uses of this
Technical Guide, including its applicability to petroleum hydrocarbons and other potentially
biodegradable chemicals and to nonresidential buildings; identifies key technical resources that
facilitate consideration of its recommendations; provides a concise historical accounting of its
development; describes how the public was involved in its development; and provides an
overview of its organization.
1.1 Definition of Vapor Intrusion
Certain chemicals that are released into the subsurface14 as liquids or solids may form
hazardous vapors that migrate or are transported through the vadose zone15 and eventually
12 The EPA Vapor Intrusion Forum is an intra-Agency group engaged in sharing information, technical resources, and
perspectives pertaining to vapor intrusion assessment and mitigation.
13 This document is intended to fulfill EPA's commitment to the OIG to issue "updated, revised, and finalized" vapor
intrusion guidance (EPA 2009a, Appendix B; EPA 201 Ob).
14 For purposes of this Technical Guide, the phrases 'released into the subsurface' and 'release to the subsurface'
are intended to encompass any and all mechanisms by which chemical contamination arises in the subsurface,
including, for example, spills and releases above the ground surface that result in subsurface (e.g., soil and
groundwater) contamination.
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enter buildings as a component of a gas16 by migrating (being transported) through cracks,
seams, interstices, and gaps in basement floors, walls, or foundations ("adventitious openings"),
through intentional openings (e.g., perforations due to utility conduits, sump pits), and/orwithin
conduits (e.g., drain and sewer lines). Vapor intrusion is the general term given to migration of
hazardous vapors from any subsurface contaminant source, such as contaminated soil or
groundwater or contaminated conduit(s), into an overlying building or unoccupied structure via
any opening or conduit.
Recognition of soil vapor intrusion to buildings and other enclosed spaces occurred in the 1980s
with concerns over radon intrusion.17 Subsequently, there was an increasing awareness that
anthropogenic chemicals (e.g., petroleum hydrocarbons and chlorinated solvents) in soil and
ground water could also pose threats to indoor air quality via the vapor intrusion pathway (Little
et al. 1992; Moseley and Meyer 1992).
Vapor intrusion can occur in a broad range of land use settings, including residential,
commercial, and industrial, and affect buildings with virtually any foundation type (e.g.,
basement, crawl space(s), or slab on grade). In the last 20 years, vapor intrusion impacts have
been demonstrated in occupied buildings at a number of sites across the country (e.g., Little et
al. 1992). As a result, vapor intrusion is widely recognized as a potential pathway of human
exposure to "volatile" hazardous chemicals in indoorspaces. When and where vaporintrusion
occurs, concentrations of vapors can increase gradually in amount in buildings or structures as
time passes (i.e., "accumulate"). Depending upon site-and building-specific circumstances,
vapors of potentially toxic chemicals may accumulate to a point where the health of the
occupants (e.g., residents, workers, etc.) in those buildings could be threatened.
In addition to their toxicity threats, methane and certain other vapor-forming chemicals can pose
explosion hazards depending upon structure-, building-, and site-specific circumstances.
Explosion hazards may pose an imminent and substantial dangerto human health and public
welfare.
Careful consideration of the vapor intrusion pathway is warranted at all sites where vapor-
forming chemicals are present in the soil or groundwateraquifer (NRC 2013).
Section 2.0 describes the vapor intrusion pathway in greaterdetail.
15 The Vadose zone' is the soil zone between land surface and the groundwater table within which the moisture
content is less than saturation (except in the capillary fringe). It is also referredto as the "unsaturated zone."
16 The terms 'gas' and Vapor' refer the gaseous state, as distinguished from the liquid or solid state, of matter.
Whereas "vapor' refers to a volatile chemical that may comprise only a portion of the total volume, 'gas' refers to the
entire volume. For economy of words, this Technical Guide refers to vapor concentrations in soil gas as "soil gas
concentrations."
17 Radon is a colorless, odorless, radioactive gas that is formed from the decay of radium, a radioactive element that
occurs naturally in the soil and bedrock in many areas of the United States. Radon can also be emitted from certain
uranium- or radium-containing products and wastes. For more information about radon, see:
http://www.epa.gov/radon/index.html.
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1.2 Statutory Authorities
Protection of human health is a critical mandate underlying several federal statutes, including
the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), as
amended,18 and the Resource Conservation and Recovery Act (RCRA), as amended.19
Protection of human health is also a critical objective of the National Oil and Hazardous
Substances Pollution Contingency Plan (NCP), which is the federal government's blueprint for
responding to oil spills and releases of hazardous substances, pollutants, or contaminants.
The sources of authority and requirements for addressing subsurface vapor intrusion are the
relevant statutes and regulations. On this basis, the EPA has broad authority and distinct
responsibilities20 to assess and, if warranted, mitigate vapor intrusion in residential and
nonresidential21 settings arising from a chemical release that causes subsurface contamination
by volatile hazardous chemicals.22 These actions may include sampling indoorair to assess
exposure levels of building occupants to subsurface vapors and implementing interim mitigation
measures to control, reduce, or eliminate exposure indoors to vapors emanating from
subsurface vaporsources. Where such subsurface contamination includes vapor-forming
chemicals (see Section 3.1) and underlies or is near buildings, EPA recommends that the
potential for human health risk from vapor intrusion be evaluated throughout the cleanup life
18
Amendments to CERCLA include the Small Business Liability Relief and Brow nfields Revitalization Act.
19
Application of these statutory authorities to a particular situation generally entails site- and fact-specific analysis.
20 On January 23, 1987, the President of the United States signed Executive Order 12580 entitled, "Superfund
Implementation," which delegates to a number of Federal departments and agencies the authority and responsibility
to implement certain provisions of CERCLA. The policies and procedures for implementing these provisions (e.g.,
carrying out response actions) are spelled out in the NCP. The provisions of Executive Order 12580 appear at 52
Federal Register 2923.
The EPA and the Occupational Safety and Health Administration (OSHA) of the Department of Labor each have a
distinct statutory responsibility to ensure the safety and health of America's workforce through the timely and effective
implementation of a number of federal laws and implementing regulations. On November 23,1990, the Secretary of
the Department of Labor and Administration of the EPA signed a Memorandum of Understanding (MOU) with the
goal of establishing a program for improved environmental and workplace health and safety, which continues in
effect. Implementation of the MOU is intended "to improve the combined efforts of the agencies to achieve protection
of workers, the public, and the environment at facilities subject to EPA and OSHA jurisdiction; to delineate the general
areas of responsibility of each agency; and to provide guidelines for coordination of interface activities between the
two agencies with the overall goal of identifying and minimizing environmental or workplace hazards." An additional
MOU was signed in February 1991 to establish a process and frameworkfor notification, consultation and
coordination between EPA and OSHA to aid both agencies in identifying environmental and workplace health and
safety problems and to more effectively implement enforcement of their respective national environmental and
workplace statutes. For additional information, see
https://www.osha.gov/pls/oshaweb/owasrch.search form?p doc type=MOU&p toe level=1&p kevvalue=Agencv&p
status=CURRENT
EPA's recommended approach for evaluating vapor intrusion exposures is based upon its existing risk assessment
guidance, as summarized in Section 7.4.
22 Section 3.1 of this Technical Guide describes technical criteria for identifying which specific chemicals are
sufficiently volatile and hazardous to generally warrant routine evaluation during vapor intrusion assessments, when
they are present as subsurface contaminants. These sufficiently volatile and hazardous chemicals are referred to as
"vapor-forming chemicals" for purposes of this Technical Guide.
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cycle (i.e., initial site assessment, site investigation, interim and final response actions,23 and
periodic reviews of the selected remedy), as described in Sections 5 and 6 of this Guide.
Although this Technical Guide is intended for use at any site subject to federal statutes,
regulations, and rules, it is not intended to alter existing requirements, guidance, or practices
among OSWER's programs about development, selection, or documentation of final
remediation24 plans (addressing subsurface vaporsources, for example).
EPA may need access to private property to conduct investigations, studies and response
actions pursuant to CERCLA and RCRA, as amended. The Superfund Amendments and
Reauthorization Act of 1986 and RCRA explicitly grant EPA the authority to enter property for
these purposes (EPA 1986, 1987,201 Oa). EPA generally prefers to obtain access through
consent and cooperation. If consent is denied, however, EPA can use the judicial process or an
administrative order to gain access. Application of legal doctrines to a particularaccess situation
warrants site- and fact-specific analysis.
Provisions under CERCLA, RCRA, federal regulations, and federal guidance also provide
authority and support for taking early actions to mitigate actual and potential human health risks,
as discussed below. In the context of vapor intrusion, "early action" may include response
measures such as engineered exposure controls to reduce or eliminate vapor intrusion in
buildings (i.e., "mitigate" vapor intrusion) or reduce indoorair exposure levels (see Sections 7.8
and 8.2) and 'prompt' response actions to address more urgent threats to human health or
public welfare (see Section 7.5).
1.2.1 Taking Action with Limited Data under CERCLA and the NCP
CERCLA and the NCP both contain provisions that support and encourage taking early actions
to mitigate actual and potential threats to human health associated with vapor intrusion. For
example, CERCLA sections 104 and 106 provide the federal government with broad authority to
take response action(s) to address a release or threatened release of hazardous substances
that "may present" a human health risk. Similarly, the preamble to the final NCP issued in the
Federal Register on March 8,1990 (55 FR 8704), states, "EPA expects to take early action at
sites where appropriate, and to remediate sites in phases using operable units as early actions
to eliminate, reduce or control the hazards posed by a site or to expedite the completion of total
site cleanup. In deciding whether to take early actions, EPA balances a number of
considerations, including the desire to definitively characterize site risks and analyze alternative
remedial approachesfor addressing those threats in great detail with the desire to implement
protective measures quickly. EPA intends to perform this balancing with a bias for initiating
The words "response action" or "response" are used generically in this Technical Guide to include remedial and
removal actions under CERCLA as amended and similar actions under RCRA as amended.
For purposes of this Technical Guide, "remediation" is intended to apply to interim and final cleanups, whether
conducted pursuant to RCRA corrective action, the CERCLA removal or remedial programs, or using EPA brownfield
grant funds with oversight by state and tribal response programs. In addition to permanent remedies for subsurface
vaporsources, site remediation may also entail implementation of institutional controls and construction and
operation of engineered systems.
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response actions necessary or appropriate to eliminate, reduce, or control hazards posed by a
site as early as possible."25
For sites that are not on the National Priorities List (NPL), EPA may use its removal authority
under CERCLA to undertake early action to mitigate vapor intrusion threats. For sites that are
on the NPL, EPA's Superfund program may use its remedial or removal authority under
CERCLA to undertake early action to ensure the protection of human health during existing or
future property uses that could be affected by vapor intrusion. Building mitigation, control of
subsurface vaporsource(s), and associated ICs could be part of a final remedy selected for the
site, or where appropriate, could represent an early action that (1) is evaluated and selected on
a faster track and (2) complements the anticipated final remedial action for the site.
Because of state cost-share consequences, EPA recommends that state concurrence be
sought for any Fund-lead mitigation under CERCLA where there is a reasonable expectation
that the state will need to take over responsibility for operations and maintenance (O&M) as part
of a long-term, final remedy.
EPA's guidance for preparing Superfund decision documents states: "An interim action is limited
in scope and only addresses areas/media that also will be addressed by a final site/operable
unit ROD [Record of Decision].... Early actions can be taken throughout the RI/FS [remedial
investigation/feasibility study] process to initiate risk reduction activities.... "Early" in this case is
simply a description of when the action is taken in the Superfund process. Thus, an early action
is one that is taken before the RI/FS for the site or operable unit has been completed. Hence,
early actions may be either interim or final" (EPA 1999b). The primary goals of an early action
are to "achieve prompt risk reduction and increase the efficiency of the overall site response"
(EPA 1992b). Although preparation of an RI/FS Report is not essential for an early action,
documentation that supports the rationale for the action and becomes part of Administrative
Record is recommended, consistentwith the NCP and CERCLA. For interim actions, EPA's
guidance for preparing Superfund decision documents states: "A summary of site data collected
during field investigations should be sufficient to document a problem in need of response. In
addition, a short analysis of remedial alternatives considered, those rejected, and the basis for
the evaluation (as is done in a focused FS) should be summarized to support the selected
action" (EPA 1999b).
For response actions selected in an Action Memorandum or Record of Decision which are
carried out by potentially responsible parties (PRPs), and where the PRP(s) agree to implement
preemptive mitigation (PEM) for vapor intrusion, EPA recommends that PRP commitments to
proceed with response action (including early action) be obtained through settlements or other
enforcement documents (for example, administrative orders). Such response action
commitments could include performance of O&M and monitoring. EPA recommends that
settlement with PRPs concerning PEM/early action response actions specify that PRPs agree
not to challenge the basis of the response based on inadequate characterization.
So, for example, EPA cited the NCP in its Compilation of Information Relating to Early/Interim Actions at Superfund
Sites and the TCEIRIS Assessment (EPA 2014b).
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1.2.2 Taking Action with Limited Data under RCRA Corrective Action
EPA has emphasized the importance of interim actions and site stabilization in the RCRA
corrective action program to control or abate "ongoing risks" to human health and the
environment while site characterization is underway or before a final remedy is selected (see
Vne Federal Register oi May 1,1996 [61 FR 19446]). Interim actions encompass a wide range of
institutional and physical corrective action activities to achieve stabilization and can be
implemented at any time during the corrective action process. EPA recommends that interim
actions, including PEM, be employed as early in the corrective action process as possible,
consistent with the human health and environmental protection objectives and priorities for the
site. EPA recommends that, as further information is collected, program implementers continue
to look for opportunities to conduct additional interim response actions.
1.3 Scope and Recommended Uses of this Technical Guide
This Technical Guide presents EPAs current recommendations for howto identify and consider
key factors when assessing vapor intrusion, making risk managementdecisions, and
implementing mitigation pertaining to this potential human exposure pathway. This Technical
Guide and the accompanying Technical Guide For Addressing Petroleum Vapor Intrusion At
Leaking Underground Storage Tank Sites (EPA 2015b) supersede and replace EPAs Draft
Guidance for Evaluating the Vapor Intrusion to Indoor Air Path way from Groundwater and Soils
(EPA 2002c) ("Draft VI Guidance"). One of the main purposes of this Technical Guide is to
promote national consistency in assessing the vapor intrusion pathway.26 At the same time, it
provides a flexible science-based approach to assessment that accommodates the different
circumstances (e.g., stage of the cleanup process) at a site and differences among pertinent
EPA programs.
This Technical Guide is intended for use at any site27 being evaluated by EPA pursuant to
CERCLA or RCRA corrective action, EPAs brownfield grantees, or state agencies acting
26 If EPA staff wish to consider using any specific guidance that is not explicitly recommended in this Technical Guide,
they should consult with Headquarters.
27 The term "site" is used generically in this Technical Guide to represent areas of contamination managed in a
cleanup project under CERCLA as amended, under RCRA as amended, at a federal facility, or pursuant to an EPA
Brow nfields grant.
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pursuant to CERCLA or an authorized RCRA corrective action program28 where vapor intrusion
may be of potential concern. EPA recommends consideration of this Technical Guide when:
Making "Current Human Exposures Under Control" environmental indicator (El)
determinations at RCRA corrective action facilities (EPA 1999a, 2002b)29 and National
Priorities List (NPL) sites under CERCLA (EPA 2008b);
Undertaking removal actions, remedial actions, pre-remedial investigations,30 remedial
investigations, and five-year reviews (FYRs)31 and selecting remedies under CERCLA;
and
Undertaking RCRA facility investigations and corrective actions and site investigations
and cleanups at federal facilities and brownfield sites.
This Technical Guide addresses both residential and nonresidential buildings that may be
impacted by vapor intrusion from subsurface vapor sources.
The broad concepts of this Technical Guide generally may be appropriate when evaluating any
of a large number and broad range of vapor-forming chemicalsdescribed in Section 3.1
that potentially can provide subsurface sources forvapor intrusion into buildings. These
chemicals include, for example, chlorinated hydrocarbons (CHCs), petroleum hydrocarbons,
other types of both halogenated and non-halogenated volatile organic compounds (VOCs),
28 EPA believes that states, tribes, and local governments will find this Technical Guide useful for their respective
programs. EPA recommends that state agencies that have delegated authority to implement CERCLA or RCRA
consider this Technical Guide when implementing their state-specific guidance for vapor intrusion assessment and
mitigation, if any, (e.g., ensure they incorporate features such as: using multiple lines of evidence to support pathway-
incomplete determinations and "no-further-action" decisions; collecting multiple rounds of indoor air sampling to
characterize exposure levels in indoor air in existing buildings and reduce the chance of reaching a false-negative
conclusion (i.e., concluding exposure is at an acceptable risk level when it is not) or a false-positive conclusion (i.e.,
concluding vapor intrusion poses unacceptable human health risk, when it does not); focusing lab analyses of indoor
air, ambient air, and sub-slab soil gas samples on vapor-forming chemicals know nor suspected to be released to the
subsurface environment; invoking the recommended criteria described in Section 6.5.2 as a condition for using risk-
based screening levels forvapor intrusion; assessing human health risk posed by less-than-chronic exposure
durations; and considering reasonably expected future conditions, as well as current conditions, when making risk
management decisions and selecting cleanup and building mitigation plans).
29 Also see http://www.epa.gov/osw/hazard/correctiveaction/eis/faqs.htm.
CERCLA authorizes the EPA to identify and prioritize which sites warrantfurther investigation to ascertain whether
remedial action is needed. The Hazard Ranking System (MRS) is the statutorily required method for evaluating and
identifying sites for placement on the NPL.
31 Section 121 of CERCLA specifies that remedial actions that result in any hazardous substances, pollutants, or
contaminants remaining at the site be re-evaluated every five years to ensure that the remedy is and will continue to
be protective of human health and the environment. OSWER Drective 9200.2-84 (Assessing Protectiveness at Sites
for Vapor Intrusion: Supplemental Guidance to the Comprehensive Five-Year Review Guidance (EPA 2012c))
provides supplemental guidance for considering vapor intrusion while evaluating remedy protectiveness in the context
of the Superfund five-year review process (even if vapor intrusion was not addressed as part of the original remedial
action).
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elemental mercury, and radon when it arises from uranium- or radium-bearing solid wastes in
the subsurface.32
This Technical Guide addresses risk management (e.g., exposure control or avoidance
methods) for indoor air contamination that arises from vapor intrusion from subsurface sources
of these vapor-forming chemicals. It is not intended as a guide for assessing or mitigating indoor
air exposures that arise solely from other sources (e.g., indoor use and storage of certain
consumer products33).
The exposure route of general interest for vapor intrusion is inhalation34 of vapors present in
indoor air that have entered via soil gas entry from the subsurface.35 Other human exposure
routes that may warrant consideration during site investigations of subsurface contamination
(e.g., ingestion of soil or water, dermal contact with soil orwater, inhalation of particulate
material, inhalation of vapors while outdoors, and inhalation of vapors while showering or
washing with contaminated groundwaterwhile indoors) are not addressed in this Technical
Guide.
EPA recommends that risk management and response action decisions for the vapor intrusion
pathway generally consider reasonably expected future conditions, which may differ from
current conditions due to changes in land use, building and infrastructure construction and
conditions, and vadose zone hydrology and oxygenation, among otherfactors. This Technical
Guide provides general information regarding howthese factors may enhance or impede vapor
intrusion. It also provides recommendations for institutional controls and monitoring where a
subsurface vaporsource(s) is(are) present and has the potential to pose unacceptable human
health risks.
Although this Technical Guide is intended for use at any site subject to federal statutes,
regulations, and rules, it is not intended to alter existing requirements, guidance, or practices
among OSWER's programs about circumstances for reviewing past risk management and
cleanup decisions. As noted, remedy reviews are required by Section 121 ofCERCLAwhen
32 Radon emanating from natural geological materials may also affect indoor air quality in occupied buildings, but is
not a subject of this Technical Guide. According to EPA estimates, inhalation of toxic radon decay products is the
leading cause of lung cancer among non-smokers. For more information about radon emanating from natural
geological materials, see: http://www.epa.gov/radon/index.html.
33 Indoor air in most buildings will contain detectable levels of a number of volatile compounds, whether or not the
building overlies a subsurface source of vapor-forming chemicals (EPA 2011 a). As discussed further in Section 2.7 of
this Technical Guide, these chemicals originate from indoor uses of chemical-containing products (e.g., household or
consumer products) and from outdoor (ambient) air. EPA's indoor air quality program provides useful advice for
control of indoor air exposures (see http://www.epa.gov/iaq/).
34 Among human exposure pathways involving contamination of land and water, vapor intrusion is distinct. Whereas
contact with contaminated surfacesoil, contaminated fish, and contaminated drinking w ater generally can be readily
avoided for prolonged periods, breathing cannot.
35 In addition, certain hazardous chemicals (e.g., methane) can pose explosion hazards when they gradually increase
in amount in structures (e.g., confined spaces) or buildings as time passes to a point where there is an imminent and
substantial danger to human health and public welfare.
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hazardous substances remain on site.36 EPA's other land restoration programs (e.g., RCRA
corrective action, brownfield redevelopment) will continue to rely upon their existing, respective
practices to address the need, if any, for periodic reviews of cleanup decisions, including
consideration of the vapor intrusion pathway.
Finally, this Technical Guide does not aim or intend to:
Offer recommendations for vapor intrusion assessments that private parties choose to
conduct as part of real estate transactions;
Modify existing guidance regarding landowner liability protection (e.g., all appropriate
inquiries, the bona fide prospective purchaserprovision); or
Offer recommendations for responding to leaks from natural gas transmission lines.
1.3.1 P etroleum Hydrocarbons
The approaches in this Technical Guide are recommended for evaluating the vapor intrusion
pathway pursuant to CERCLA or RCRA corrective action for petroleum hydrocarbons that are
mixed with other types of volatile hazardous chemicals or are the result of releases from
sources other than Subtitle I underground storage tank (LIST) systems.37 For petroleum
hydrocarbons that arise from petroleum that has been released from Subtitle I LIST systems,
EPA has developed a companion to this Technical Guide (Technical Guide ForAddressing
Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites (EPA 2015b)), which
provides information and guidance about assessing vapor intrusion from petroleum
hydrocarbons in these settings and may also be useful in informing decisions about vapor
intrusion and petroleum hydrocarbons at non-UST sites that are similar in size to a typical
Subtitle I LIST release.
Many petroleum hydrocarbons may naturally biodegrade in the vadose zone through the actions
of microorganisms found naturally in soil. When oxygen supply from the atmosphere is
sufficient, biodegradation of petroleum hydrocarbons can occur relatively quickly, will generally
produce less harmful compounds (i.e., biodegradation products), and can result in substantial
attenuation of petroleum hydrocarbon vapors over relatively short distances in the vadose zone.
Numerous site-specific factors can influence the biodegradation rate of petroleum hydrocarbons
(and other biodegradable vapor-forming chemicals) in the vadose zone. These factors include
quantities, distribution, types, and mixtures of vapor-forming chemicals, which can differ
substantially among sites where petroleum hydrocarbons are released to the subsurface
36 The NCR states [40 CFR 300.430(f)(4)(ii)]: "If a remedial action is selected that results in hazardous substances,
pollutants, or contaminants remaining at the site above levels that allow for unlimited use and unrestricted exposure,
the lead agency shall review such action no less often than every five years after initiation of the selected remedial
action." For further information, see, for example, http://www.epa.aov/superfund/cleanup/postconstruction/5vr.htm
For example, the approaches in this Technical Guide are recommended for evaluating the vapor intrusion pathway
associated with subsurface releases of petroleum, petroleum derivatives, and petroleum hydrocarbons from
refineries, bulk storage facilities, oil exploration and production sites, pipelines and transportation, chemical
manufacturing facilities, former manufactured gas plants, creosote (wood-treating) facilities, large-scale fueling and
storage operations at federal facilities, and dry cleaners that use petroleum solvents.
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environment. This Technical Guide allows site-specific observations of the effects of
biodegradation to be considered in its approach for petroleum hydrocarbons (and any other
biodegradable, vapor-forming chemical). Sections 6.3.2 and 7.3 expand on EPA's
recommended approach to evaluating biodegradation of vapor-forming chemicals in the vadose
zone at sites with subsurface contamination.
1.3.2 Nonresidential Buildings
EPA has broad authority and distinct responsibilities to assess and, if warranted, mitigate vapor
intrusion in nonresidential settings arising from a chemical release that causes subsurface
contamination by volatile hazardous chemicals (see Section 1.2). EPA38 is authorized to take all
appropriate actions to protect human health and the environmentfrom subsurface vapor
sources of chemical exposure consistent with applicable federal statutes39'40 and regulations
and considering EPA guidance,41 taking into account the nonresidential setting. These actions
may include sampling indoor air to assess exposure levels of building occupants to subsurface
vapor sources and implementing interim mitigation measures to control, reduce, or eliminate
exposure indoors to vapors emanating from a subsurface vapor source(s).
As used in this Technical Guide, the phrase "nonresidential buildings" may include, but is not
limited to, institutional buildings (e.g., schools, libraries, hospitals, community centers and other
enclosed structures for gathering, gyms and other enclosed structures for recreation);
commercial buildings (e.g., hotels, office buildings, many (but not all) day care facilities, and
retail establishments); and industrial buildingswhere vapor-forming chemicals may or may not
be routinely used or stored. Section 4.0 expands on EPA's recommended approach to
evaluating and mitigating vapor intrusion in nonresidential buildings.
1.4 Companion Documents and Technical Re sources
Technical information pertaining to vapor intrusion has also been prepared to support
development of and facilitate implementation of the technical approaches and recommendations
in this Technical Guide. Key technical information is described in this section and can be found
on OSWER's vapor intrusion website (see Section 11.0 for citations and internet links).
38 On January 23, 1987, the President of the United States signed Executive Order 12580 entitled, "Superfund
Implementation," which delegates to a number of Federal departments and agencies the authority and responsibility
to implement certain provisions of CERCLA. The policies and procedures for implementing these provisions (e.g.,
carrying out response actions) are spelled out in the NCR The provisions of Executive Order 12580 appear at 52
Federal Register 2923. Atfederal facilities on the NPL, EPA may not be the lead agency, but does have oversight
responsibilities pursuant to CERCLA Section 120.
CERCLA and RCRA authorize EPA to protect human health and the environment, as summarized in Section 1.2 of
this Technical Guide. The NCP also addresses protection of human health and the environment.
40 See, for example, CERCLA Section 101(22).
See, for example, OSWER Directive 9355.0-30 (Role of the Baseline Risk Assessment in Superfund Remedy
Selection Decisions) (EPA 1991 a) and Rules of Thumb for Superfund Remedy Selection, OSWER Drective 9355.0-
69, August 1997 (EPA 1997).
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1.4.1 Vapor Intrusion Screening Level Calculator
The Vapor Intrusion Screening Level (VISL) Calculator (2015a) is a technical resource,
developed by EPA that:
(1) Identifies chemicals considered to be typically vapor-forming and known to pose a
potential cancer risk or noncancer hazard through the inhalation pathway (as described
further in Section 3.1 herein);
(2) Provides generally recommended sere en ing-1 eve I concentrations for groundwater, near-
source soil gas (exterior to buildings), sub-slab soil gas, and indoor air based upon
default residential or nonresidential exposure scenarios, a target cancer risk level of one
per million (10"6), and a target hazard quotient of one for potential non-cancer effects;
and
(3) Facilitates calculation of site-specific screening levels (see Section 6.5) and/or candidate
cleanup levels (see Section 7.6) based on user-defined target risk levels, exposure
scenarios, and semi-site-specific (AppendixA) or site-specific (Section 7.6) attenuation
factors.
The VISL Calculator is comprised of an MS Excel workbook. It can be used in evaluating
whether the vapor intrusion pathway has the potential to pose a human health risk by helping to:
(1) Identify whether volatile hazardous chemicals that can pose a threat through vapor
intrusion are present;
(2) Determine if those volatile hazardous chemicals are present at potentially explosive
levels;
(3) Compare subsurface or indoor data against recommended screening levels provided in
the VISL Calculator; and
(4) Prioritize buildings and sites for investigation and response action.
The recommended screening-level concentrations in the spreadsheet are calculated using the
recommended approaches in existing EPA guidance for human health risk assessment, as
described furtherin Sections 6.5.2 and 6.5.3 herein, and are based on current understanding of
the vapor intrusion pathway.
1.4.2 Technical Support Documents
Key technical documents supporting development of the technical approaches and
recommendations in this Technical Guide include:
Background Indoor Air Concentrations of Volatile Organic Compounds in North American
Residences (1990-2005): A Compilation of Statistics for Assessing Vapor Intrusion (EPA
2011a): This technical report presents (1) a summary of indoor air studies that measured
background concentrations of VOCs in the indoor air of thousands of North American
residences and (2) an evaluation and compilation of the statistical information reported in
these studies. The objective of this compilation is to illustrate the ranges and variability of
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VOC concentrations in indoor air during the study period (1990-2005), resulting from
sources other than vaporintrusion. This technical report was externally peer reviewed,
consistent with EPA's peer review policy (EPA-SPC 2006) for scientific and technically
based work products that are intended to inform Agency decisions.
EPA's Vapor Intrusion Database: Evaluation and Characterization of Attenuation Factors for
Chlorinated Volatile Organic Compounds and Residential Buildings (EPA 2012a): This
technical report presents technical information about sites in the U.S. that have been
investigated for vapor intrusion. The primary focus of the report is the evaluation of
concentrations of chlorinated VOCs in and underneath residential buildings based upon the
EPA's vapor intrusion database as of 2010. This report provides the technical basis forthe
generic and semi-site-specific attenuation factors recommended in this Technical Guide to
calculate vapor intrusion screening levels (see Section 6.5 and AppendixA). This technical
report was externally peer reviewed, consistent with EPA's peer review policy (EPA-SPC
2006) for scientific and technically based work products that are intended to inform Agency
decisions.
Conceptual Model Scenarios for the Vaporintrusion Pathway (EPA 2012b): This technical
report provides simplified simulation examples to illustrate graphically how subsurface
conditions and building-specific characteristics determine: (1) the distribution of vapor-
forming chemicals in the subsurface; and (2) the indoor air concentration relative to a source
concentration. It was prepared to help environmental practitioners gain insights into the
processes and variables involved in the vaporintrusion pathway and to provide a theoretical
framework with which to draw inferences about and better understand the complex vapor
fate and transport conditions typically encountered at actual, contaminated sites. This
technical report was externally peer reviewed, consistent with EPA's peer review policy
(EPA-SPC 2006) for scientific and technically based work products that are intended to
inform Agency decisions.
These technical tools and documents, as well as others, can be found at
http://www.epa.gov/oswer/vaporintrusion, a website developed to support the development of
this Technical Guide and enhance public communication about the topic. This website also
allows certain sections of this Technical Guide to be more dynamic and facilitates updates to
information.
Technical documents intended to facilitate consideration of the recommendations in the
Technical Guide For Addressing Petroleum Vaporintrusion At Leaking Underground Storage
Tank S/tes(EPA2015b) can be found at http://www.epa.gov/oust/cat/pvi/.
1.5 Historical Context
To help assess the subsurface vapor intrusion pathway, the Office of Solid Waste and
Emergency Response (OSWER) released in November 2002 for comment EPA's Draft VI
Guidance, which presents EPA's technical information and recommendations for evaluating
subsurface vapor intrusion, based on the understanding of vapor intrusion at that time (EPA
2002c). This Technical Guide and the accompanying Technical Guide For Addressing
Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites (EPA 2015b) supersede
and replace the Draft VI Guidance.
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Since the Draft VI Guidance was released, EPA's knowledge of and experience with
assessment and mitigation of the vapor intrusion pathway has increased considerably, leading
to an improved understanding of and enhanced approaches for evaluating and managing vapor
intrusion. In addition:
Other federal agencies with responsibilities and obligations for environmental cleanup or
for response to reports of vapor intrusion have developed vapor intrusion guides for their
respective programs (e.g., ATSDR 2008; DoD 2009; DoN2011a; USPS2009).
A number of state agencies involved with environmental quality or public health
protection have developed vapor intrusion guides for their programs, which they may
continue to implement under their respective statutory authorities (e.g., see ASTSWMO
[2009], a compilation).
The Interstate Technology & Regulatory Council (ITRC), a state-led coalition of
environmental regulatory professionals, prepared three guidelines for assessing the
vapor intrusion pathway (ITRC 2007ab, 2014).
EPA has considered these guides in developing this Technical Guide.
In addition, in December 2009, the OIG made recommendations regarding EPA's Draft VI
Guidance, which are documented in the evaluation report Lack of Final Guidance on Vapor
Intrusion Impedes Efforts to Address Indoor Air Risks (Report. No. 10-P-042; EPA2009a).
Among other things, the OIG recommended that the final guidance incorporate:
Updated toxicity values.
A recommendation(s) to collect and weigh multiple lines of evidence in evaluating and
making decisions about human health risks posed by vapor intrusion.
How risks from petroleum hydrocarbon vapors should be addressed.
How the guidance applies to Superfund FYRs.
When or whether preemptive mitigation is appropriate.
Operations, maintenance, and termination of mitigation systems.
When institutional controls are appropriate.
In its response letter dated March 11, 2010, OSWER generally agreed with OIG's
recommendations to finalize guidance on vapor intrusion. In addition, the OIG recommended
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that EPA identify and publicly report the portions of its Draft VI Guidance that remain valid and
the portions that would be updated.42
This Technical Guide and the companion documents identified in Sections 1.3 and 1.4 fulfill
EPA's commitment to address the OIG's recommendations. Table 1-1 identifies specific
updates prepared by EPA in response to OIG's specific recommendations. Table 1-2 describes
additional updates identified and publicly announced by EPA (EPA 201 Ob).
TABLE 1-1
DIRECTORY TO UPDATES IN THIS TECHNICAL GUIDE ADDRESSING
RECOMMENDATIONS OF EPA OFFICE OF INSPECTOR GENERAL (EPA 2009A)
Topics to Be Addressed
Location Within
This Technical
Guide
Companion Documents
Update toxicity values
VISL Calculator (EPA2015a)
Use of multiple lines of evidence in evaluating
and making decisions about risks from vapor
intrusion
Sections 5, 6, and
7
How risks from petroleum hydrocarbon vapors
should be addressed
How this Technical Guide applies to Superfund
Five-year Reviews (FYRs)
When or whether preemptive mitigation/early action
is appropriate
Operations and maintenance of mitigation systems
Termination of mitigation systems
When ICs and deed restrictions are appropriate.
Sections 1.3.1,
6.3.2 and 7.3
Sections 3. 3 and
7.8
Section 8.3
Section 8.7
Section 8.6
Technical Guide for
Addressing Petroleum Vapor
Intrusion at Leaking
Underground Storage Tank
S/tes(EPA2015b)
Assessing Protectiveness at
Sites for Vapor Intrusion:
Supplemental Guidance to the
Comprehensive Five-Year
Review Guidance (EPA
2012c)
42
OSWER carried out this recommendation by issuing a memorandum in August 2010, entitled Review of the Draft
2002 Subsurface Vapor Intrusion Guidance (EPA 201 Ob). The guidance reflected in this memorandum is
incorporated in this Technical Guide.
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TABLE 1-2
DIRECTORY TO ADDITIONAL UPDATES IN THIS TECHNICAL GUIDE PUBLICLY
IDENTIFIED BY OSWER (EPA 2010B)
Topics to Be Updated, Including References to the Draft VI
Guidance
Location Within This
Technical Guide
Com panion Technical
Documentor Resource
Updated a few chemical-specific physical parameters used for
identifying the vapor-forming chemicals of concern.
Section 3.1
VISL Calculator (EPA
2015a)
Updated thetoxicity-based criteria in Table D-1 in the draft
guidance.
Section 3.1
VISL Calculator (EPA
2015a)
Observation-based conservative attenuation factors have been
updated with a larger database. The generic attenuation factor
for external soil gas has been updated, as well as the
Reliability Assessment, using the newer available data.
Section 6.5.3 and
Appendix A
US. EPA's Vapor
Intrusion Database:
Evaluation of Attenuation
Factors for Chlorinated
Volatile Organic
Compounds and
Residential Buildings
(EPA 2012a)
Observational data since 2002 indicates that the "single line of
evidence" approach with site-estimated attenuation factors is
generally not appropriate for external soil gas samples.
Section 6.4.4 and
Appendix A
Experiences since 2002 illustrate the value of collecting indoor
air samples earlier in the investigations. The "indoor air last"
approach has been updated, which will allow more flexibility in
the sequencing of subsurface and interior/indoor sample
collection.
Sections 6.3.4 and
6.3.6
The portions addressing background contamination have been
updated. EPA also updated with more specific methodologies
for evaluating and/or decision-making and managing
background contamination.
Sections 6.3.5, 7.4 and
7.6
Background Indoor Air
Concentrations of Volatile
Organic Compounds in
North American
Residences (1990-2005):
A Compilation of Statistics
for Assessing Vapor
Intrusion (EPA 2011 a)
The portion of this Technical Guide focusing on testing indoor
air has been updated to allow more flexibility in the duration of
sampling to take advantage of other sampling durations and
methods.
Section 6.4.1
The Draft VI Guidance allows site-specific decisions to be
made based on indoor air concentrations in a relatively few
representative buildings. This portion of this Technical Guide
has been updated to increase the confidence that the
approach fully addresses building-by-building variability.
Sections 6.2.2 and 7.8
Updated and expanded the community involvement
information to be more specific to vapor intrusion sites,
including guidelines for effective risk communication and
available resources, outreach products and tools for outreach.
Section 9
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1.6 Public Involvement in Developing this Vapor Intrusion Technical Guide
On November 29, 2002, EPA published a notice in the Federal Register (67 FR 71169)
announcing and soliciting comment on its Draft VI Guidance. Since that time, EPA continued to
gather information and learn more aboutvapor intrusion, in part by convening periodic forums
where practitioners, regulated parties, and regulators could discuss the emerging science and
engineering pertaining to vapor intrusion assessment and mitigation. In addition, on March 17,
2011, EPA published a notice in the Federal Register (76 FR 14660) re-opening the docket and
soliciting additional comment on its development efforts for this Technical Guide. The docket
was re-opened again in March 2012 to receive comments about specific technical documents
that were prepared to support development of this Technical Guide; these technical documents
are listed in Section 1.4. Finally, another review draft was released on April 16, 2013 for public
comment. In developing and refining this Technical Guide, EPA considered all public comments
and input received since 2002.
EPA also proactively engaged communities beyond the traditional outreach practices, especially
environmental justice communities and communities subject to multiple stressors.43 Aspects of
this engagement have included:
Conducting public listening sessions in communities impacted by vapor intrusion to
solicit input on developing this Technical Guide.
Using Internet sites and other communication tools to update stakeholders on the
progress of developing this Technical Guide.
Table 1 -3 identifies specific vapor intrusion topics that have received substantive public
comment as a result of EPAs outreach efforts.
1.7 Organization
The next nine sections of this Technical Guide are as follows:
Section 2.0 Conceptual Model of Vapor Intrusion further describes vapor intrusion and
identifies many of the variables that influence vapor migration in the vadose zone and
soil gas entry into buildings.
Section 3.0 Overview of this Vapor Intrusion Technical Guide provides an overview of
this Technical Guide and the general recommended framework for vapor intrusion
assessment and response action.
Section 4.0 Considerations for Nonresidential Buildings provides information regarding
EPA roles, responsibilities, and risk management decision-making in nonresidential
settings, including those (e.g., manufacturing facilities) where workers handle volatile
hazardous chemicals similar to or different from those contaminating the subsurface.
For more information about the Community Engagement Initiative visit:
http://www.epa.gov/oswer/engagementinitiative/
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TABLE 1-3
VAPOR INTRUSION TOPICS RECEIVING SUBSTANTIVE PUBLIC COMMENT
Topics
Location Within
This Technical Companion Document(s)
Guide
Applicability to petroleum hydrocarbons
Sections 1.3.1,
6.3.2 and 7.3
Technical Guide for
Addressing Petroleum Vapor
Intrusion at Leaking
Underground Storage Tank
S/tes(EPA2015b)
Applicability to nonresidential buildings
Sections 1.3.2, 4.0
and 7.4.3
Conditions warranting prompt response action
Sections 5.2, 7.5
and 8.2.1
Planning investigations and applying data quality Section 6.2 and
objectives Appendix B
Sampling and monitoring methods for indoor air
Section 6.4.1
Attenuation factors and risk-based screening
Section 6.5 and
Appendix A
U.S. EPA's Vapor Intrusion
Database: Evaluation of
Attenuation Factors for
Chlorinated Volatile Organic
Compounds and Residential
Buildings (EPA 2012a)
Semi-site-specific screening and application of
mathematical models
Sections 6.5 and
6.6
Use of conceptual site models and multiple lines of
evidence in evaluating risks posed by vapor
intrusion
Sections 2, 3.2,
5.4, 6.3 and 7
Risk management benchmarks and decision-
making
Section 7
Use of institutional controls for building mitigation Section 8.6
Monitoring and termination of mitigation systems
Sections 8.4 and
8.7
Risk communication
Section 7.4 and 9
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Section 5.0 Preliminary Analysis of Vapor Intrusion provides technical information for
situations where only limited site-specific data may be available (e.g., initial site
assessment).
Section 6.0 Detailed Investigation of Vapor Intrusion provides technical information for
conducting site-specific vapor intrusion assessments emphasizing multiple lines of
evidence, including consideration of background concentrations.
Section 7.0 Risk Assessment and Management Framework provides general
recommendations about data evaluations and risk-informed decision-making pertaining
to vapor intrusion, including consideration of background concentrations.
Section 8.0 Building Mitigation and Subsurface Remediation provides technical
information for mitigating vapor intrusion and describes how subsurface vapor source
remediation and otherfinal cleanup actions are combined with engineered and non-
engineered exposure controls to ensure protection of human health.
Section 9.0 Planning for Community Involvement provides information and describes
available resourcesfor engaging affected communities and communicating risk-related
information.
Section 10.0 Glossary provides definitions and descriptions of key terms used in this
document.
This Technical Guide concludes with Section 11.0, Citations and References, and three
supporting appendices:
Appendix A: Recommended Subsurface-to-lndoor-Air Attenuation Factors.
Appendix B: Data Quality Assurance Considerations.
AppendixC: Calculating Vapor Source Concentration from Groundwater Sampling Data.
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2.0 CONCEPTUAL MODEL OF VAPOR INTRUSION
This section presents a general (i.e., not site-spedfic) conceptual model of vapor intrusion,
borrowing from published depictions (EPA 2008a; EPA 2012b; ITRC2007a; McAlary etal.
2011; DoD2009). It identifies and describes the'what', 'where', 'how1 and'why' of vapor
intrusion, to provide insights about the many of the lines of evidence pertinent to evaluating
vapor intrusion on a site-specific basis, which are discussed further in Sections 5, 6, and 7 of
this Technical Guide.44 It concludes with several general observations that may assist
practitioners when planning and conducting detailed vapor intrusion investigations at specific
sites, which is the subject of Section 6 of this Technical Guide.
Vapor intrusion is a potential human exposure pathway a way that people may come into
contact with hazardous vapors while performing day-to-day indooractivities. Figure 2-1
summarizes the vapor intrusion pathway for soil gas entry. For purposes of this Technical
Guide, the vapor intrusion pathway is referred to as "complete"45 for a specific building or
collection of buildings when the following five conditions are met under current conditions:
1. A subsurface source of vapor-forming chemicals is present (e.g., in the soil or in
groundwater) underneath or near the building(s);
2. Vapors form and have a route along which to migrate (be transported) toward the
building(s);
3. The building(s) is(are) susceptible to soil gas entry, which means openings exist for the
vapors to enter the building and driving 'forces' exist to drawthe vapors from the
subsurface through the openings into the building(s);
4. One or more vapor-forming chemicals comprising the subsurface vaporsource(s) is (or
are) also present in the indoor environment; and
5. The building(s) is (or are) occupied by one or more individuals when the vapor-forming
chemical(s) is (or are) present indoors.46
If one (or more) of these conditions is currently absent and is reasonably expected to be absent
in the future (e.g., vapor migration is significantly and persistently impeded by natural geologic,
hydrologic, or biochemical (e.g., biodegradation) processes and conditions), the vapor intrusion
pathway is referred to as "incomplete."
44 In general, a conceptual site model integrates all lines of site-specific evidence into a three-dimensional
conceptualization of site conditions that includes contaminant sources, release mechanisms, vapor migration
route(s), and potential receptors. Section 5.4 provides additional information about developing conceptual site
models.
45 A complete vapor intrusion pathway indicates that there is an opportunity for human exposure in the subject
building(s), w hereas an incomplete pathway would not provide an opportunity forhuman exposure,
46 The exposure route of general interest for vapor intrusion is inhalation of toxic vapors present in indoor air.
Because breathing is not avoidable for prolonged periods, individuals in occupied buildings are presumed to be
exposed by the inhalation route to any hazardous vapors present in indoor air. Hence, the presence of a human
exposure route is implied in the fifth condition.
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Figure 2-1 Illustration of Key Elements of the Conceptual Model of Soil Vapor Intrusion
Note: QSOJI represents soil gas entry; Qb|dg represents air exchange.
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The first three of these five conditions are further discussed in the next three subsections.
Knowledge of potential vapor sources and vaporfate and transport mechanisms is essential for
interpreting the data collected during a site-specific investigation of vapor intrusion. Knowledge
of the factors that influence the vapor intrusion pathway is also invaluable for identifying,
prioritizing, and sequencing data collection activities, which allows a phased and efficient overall
investigation plan to be developed. Practitioners are encouraged to refer to quantitative
discussions of these subjects, which are provided in Conceptual Model Scenarios for the Vapor
Intrusion Pathway (EPA 2012b).
The human population of primary interest is comprised of individuals living in, working in, or
otherwise occupying a building subject to vapor intrusion. All types of buildings have openings
and conduits that renderthem potentially vulnerable to vapor intrusion. This includes residential
buildings (e.g., single-family homes, trailer or'mobile' homes, multi-unit apartments and
condominiums), commercial workplaces (e.g., office buildings, retail establishments), industrial
facilities (e.g., manufacturing plants), and educational and recreational buildings (e.g., schools
and gyms). Vapor intrusion can occur in buildings with any foundation type (e.g., basement,
crawl space, slab-on-grade).
As noted previously, methane and certain othervapor-forming chemicals can also pose
explosion hazards in buildings and unoccupied structures,47 depending upon building-,
structure-, and site-spedfic circumstances. The discussion in the next three sections pertains
also to methane and other vapor-forming chemicals that can pose explosion hazards, because
similar processes and conditions are involved in explosive vapors migrating towards the interior
of buildings or non-occupied structures from the subsurface environment; i.e.,
1) A subsurface source of vapor-forming chemicals is present (i.e., in the soil or in
groundwater) underneath or near the structure(s) or building(s).
2) Vapors form and have a route along which to migrate (be transported) toward the
structure(s) orbuilding(s).
3) The structure(s) or building(s) is (or are) susceptible to soil gas entry, which means
openings exist for the vapors to enter the structure(s) or building(s) and driving 'forces'
exist to draw the vapors from the subsurface through the openings into the structure(s)
orbuilding(s).
For purposes of evaluating potential explosion hazards, non-occupied structures, in addition to buildings, are
relevant structures for intrusion and accumulation of vapors.
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2.1 Subsurface Vapor Sources
The originating (i.e., primary) source(s) of subsurface contamination may include, but are not
limited to, leaking tanks (above or below ground), discharges to sewer lines48, septic tanks, and
floor drains, landfills and otherland disposal management units, fire-training areas, spills,
discharge areas, and vapor leaks from pressurized tanks and pipelines. The resulting
subsurface contamination may be comprised of non-aqueous-phase liquids (NAPLs)49 (e.g.,
solvents; petroleum-related products, such as gasoline) and contaminated soil. These are often
referred to as the source zone(s). Groundwater and sewer lines50 flowing through or
underneath51 the source zone(s) can become contaminated and in turn become a (secondary or
derivative) subsurface vapor source of contaminant vapors at locations distant from the source
zone.
Contaminants in soil, NAPLs, and groundwater can become sources for vapor intrusion if they
are likely to volatilize under normal temperature and pressure conditions. Water solubility is also
a factor for chemicals in source zones that come into contact with migrating groundwater.
Common classes of chemicals of concern for vapor intrusion that exhibit the foregoing
characteristics are VOCs, such as tetrachloroethylene (PCE), trichloroethylene (TCE), vinyl
chloride, carbon tetrachloride, and benzene, toluene, ethylbenzene andxylenes (collectively,
BTEX). Other compounds that are not as volatile as these VOCs (e.g., so-called semi-volatile
organic compounds), but that may be cause for concern, include some polycyclic aromatic
hydrocarbons (PAHs) (e.g., naphthalene), some polychlorinated biphenyl (PCB) congeners, and
elemental mercury, a dense NAPL (DNAPL).52
Historically, sanitary sewers and septic tanks have been common disposal points for aqueous and chemical wastes
from commercial and industrial operations. Contaminated water, non-aqueous phase liquid (NAPL), and VOC vapors
can leak from sewer lines through cracks, joints, or breaks. A study of solvent contamination in California arising from
dry cleaning operations concluded that discharges to and leakage from sewer lines is an important source of PCE
contamination of soil and groundwater: "Where a source investigation has been done in connection with PCE
contamination, the ... data strongly indicate that leakage through the sewer lines is the major avenue through which
PCE is introduced to the subsurface." (Izzo 1992). In the South Weber neighborhood near the Hill Air Force Base in
Utah, sewer lines carrying discharged contaminated groundwater to the municipal treatment system were identified
as a source of vapor intrusion in homes [Source: EnviroNews - Updating environmental issues and activities at Hill Air
Force Base, Utah (March 2011); Currently available on-line at: http://www.hillrab.org/news.aspx1
49
EPA's Contaminated Site Cleanup Information website(http://www.clu-in.org/) provides information describing
NAPLs that are denser than water(DNAPLs) or less dense than water (LNAPLs), and methods fortheir detection and
remediation in the subsurface environment.
50 In addition to receiving direct discharges, sewers can be indirect receptacles of subsurface contamination via
infiltration of NAPL, soil gas, or contaminated groundwater through cracks in piping and manholes. For example,
Vroblesky etal (2011) found that infiltration of contaminated groundwater into sewers and its transport via and
exfiltration from sewers caused complex and unanticipated patterns of groundwater contamination at a site in South
Carolina.
51 Figure 2-1 illustrates a NAPL release/source (near the commercial/industrial building on the left) that fully
penetrates the vadosezone. A partially penetrating NAPL release/source may also cause groundwater
contamination, however, as infiltrating water passes through the source zone and migrates to the groundwater table.
52 Once volatilized into soil or sewer gas from a subsurface vapor source(s), these less volatile chemicals will migrate
under the influence of diffusion and advection (see Section 2.2) as do more volatile chemicals, although there may be
chemical-specific differences in their susceptibility to biodegradation in thevadose zone.
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Landfill gases, such as methane and hydrogen sulfide, also can be associated with the vapor
intrusion pathway for buildings located near current or former landfills or other degrading
wastes. These gases are actively produced as a result of anaerobic biodegradation processes.
Methane can also be associated with the vapor intrusion pathway for buildings located near
degrading petroleum hydrocarbons or fuel-grade ethanol released into the subsurface
environment (Ma et al. 2014, 2012; Sihota et al. 2013).
Properties with potential contamination by vapor-forming chemicals can be found in many
industrial and commercial areas. These properties include current and former manufacturing
and chemical processing plants, warehouses, landfills and other land disposal units, coal
gasification plants, chemical handling ortransferfacilities and areas (e.g., train yards), dry
cleaners, and retail fueling outlets (also known as gas stations). Use, storage, or transport of
chemicals at these facilities may have resulted in a release of vapor-forming chemicals to the
environment creating the potential for future vapor intrusion issues. In addition to industrial and
commercial activities, roadside dumping, pesticide spraying, or even disposal of household
chemicals via a septic field may also release volatile chemicals that may eventually migrate to
the subsurface environment.
The primary contamination source need not, however, be on the property of interest to pose a
vapor intrusion problem.53 As illustrated in Figure 2-1, the primary source(s) of vapor intrusion
(e.g., contaminated soil, or leaked tanks) maybe present on a neighboring property or on a
property some distance away. Even "greenspace" properties that have not previously been
occupied or developed may contain contamination by vapor-forming chemicals due to migrating
plumes of contaminated groundwateror migrating soil gases.54
In the case of groundwater as a subsurface vapor source for vapor intrusion, the source
strength will be influenced by the vertical distribution of contaminant concentrations in the upper
reaches (e.g., top foot) of the water table and by seasonal fluctuations in the groundwater table
groundmass flux of vapors. If vapor-forming chemicals are not present in the upper reaches
(e.g., within the uppermost foot) of the groundwater table (e.g., due to the presence of an
overlying zone of clean water from recharge; i.e., "fresh water lens"),55 vapor transport to the
overlying vadose zone will be impeded due to the slower diffusion of volatile chemicals in water
Depending on the geology and amount and form of contamination in the source zone(s), contaminated
groundwater plumes can be long and narrow and can flow beneath a property located a mile or more away from the
primary source. Soil gas plumes tend to extend in both lateral directions and can be larger in lateral extent relative to
groundwater plumes.
54 See Section 6.2.1 forfurther discussion on which buildings and non-occupied structures are considered "near" for
purposes of a preliminary analysis.
Infiltrating precipitation is important in recharging aquifers with freshwater, as well as in wetting vadose zone soils.
At locations distant from "source zones," infiltrated water that reaches the upper surf ace of a plume of contaminated
groundwater (i.e., recharges groundwater) in an unconfined aquifer will tend to dilute concentrations of vapor-forming
chemicals and may form a lens of relatively "clean" water at the groundwater table, which overlies the plume.
Because diffusion of dissolved-phase volatile chemicals w ill tend to control the mass transfer of vapors into the soil
gas at the groundwater table, the presence of a lens of clean water as little as a foot in thickness overlying a plume
may be sufficient to impede vapor flux to the vadose zone (McAlary et al. 2011). This condition is less likely to occur
where fluctuations of the groundwater table are large, relative to local recharge, and would not generally be expected
in arid climates.
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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than in soil gas. For this reason, Figure 2-1 does not show vapors emanating from the leading
(i.e., right-most) edge of the plume.
If the vapor-forming chemicals are present in the upper reaches of the groundwatertable (i.e.,
volatile chemicals are in the uppermost reaches of an unconfined - "water table" - aquifer),
fluctuations in the water table will tend to transport the volatile chemicals upward (during periods
of rising water table) or expose impacted water above the water table to soil gas (during periods
of falling water table). The latter will facilitate the episodic formation of vapors in the vadose
zone. Rising water tables also will bring the vapor source closer to the building(s).
2.2 Subsurface Vapor Migration
At many sites, the subsurface vapor source (e.g., insoilorgroundwater) is not in contact with
the bottom of the subject building. Under these circumstances, vapors emanating from the
source medium enter the pore space around and between the subsurface soil particles in the
soil column above the groundwatertable, which is called the unsaturated soil zone or vadose
zone. If the subsurface vaporsource is in the vadose zone, the vapors have the potential to
migrate radially in all directions from the source via diffusion (i.e., upward toward the
atmosphere, laterally outward, and downward toward the water table; downward migration may
eventually lead to groundwatercontamination). If the subsurface vaporsource is in the upper-
most zone of groundwater, the vapors have the potential to migrate upwards toward the
atmosphere via diffusion. Figure 2-1 illustrates these conditions and this process.
Regardless of source type, soil gas concentrations emanating from a subsurface vaporsource
generally attenuate, or decrease, as the volatile chemicals move from the source through the
soil and into indoor air. If and when soil vapor monitoring data at a given site are not consistent
with this trend, the conceptual site model may be incomplete (e.g., additional, unrecognized
sources or a preferential migration route(s) may exist at the site) and/or bias or error may have
been imparted by the sampling and analysis techniques.
Diffusion, which is caused by the random motion of molecules, affects the distribution of soil
vapors when there are spatial differences in chemical concentrations in the soil gas. The net
direction of diffusive transport is toward the direction of lower concentrations.
Advection occurs in the vadose zone when there is bulk movement of soil gas induced by
spatial differences in soil gas pressure. The direction of advective vapor transport is always
toward the direction of lower air pressure. Advection is generally expected to occur in the vicinity
of buildings, because differences in temperature between the building interior and the
subsurface environment or the operation of combustion units or fans within the building can
create driving forces for soil gas entry (See Section 2.3). Advection of soil gas may also occur:
near the ground surface due to fluctuations in barometric (atmospheric) pressure, which
can either release soil gas into the atmosphere (Clements and Wilkening 1974) or
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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introduce ambient air into the subsurface environment (the latter process may be
important in oxygenating surface soil horizons);56
wherever methane generation from anaerobic degradation is sufficiently high (e.g., near
some landfills, some locations with degrading fuels).
Advection may be hindered where extensive surface barriers, such as asphalt, concrete, or
frozen soil are present.
Vapors also can migrate via advection (and diffusion) along a preferential subsurface pathway,
such as a utility corridor or more porous zones of soil or rock, or beneath surface barriers that
limit the direction(s) of vapor migration, such as frozen ground or asphalt.57
Vapor migration in the vadose zone can be impeded by several factors, including high soil
moisture, low-permeability (generally fine-grained) soil, and biodegradation:
High moisture levels in the vadose zone can significantly reduce the effective rate of
diffusive transport, owing to the substantially smaller diffusion coefficient of vapor-
forming chemicals in water compared to air. Where ground covers, such as asphalt or
concrete, are absent, soil cores taken external to building structures can reasonably be
expected to show greater soil moisture than underneath buildings (Tillman and Weaver
2007), particularly after episodes of precipitation and infiltration. Fluctuations in the
elevation of the groundwater table can also contribute to temporal changes in soil
moisture profiles, in addition to changing the thickness of the vadose zone.
A low-permeability layer in the vadose zone, particularly one with high moisture content
or perched water, may impede or prevent upward migration of vapors from deeper
sources in the vadose zone. Figure 2-1 illustrates partial impedance due to a silty or clay
layer of limited lateral extent.58 In some cases, soil or rock can impose sufficient
resistance to vapor migration to make the vapor intrusion pathway insignificant, because
low-permeability layers are laterally extensive overdistances that are large compared to
the size of the building(s) or the extent of subsurface contamination with vapor-forming
chemicals.
56 Under certain conditions, such as periods during which indoor-outdoor pressure differences are small, atmospheric
pressure fluctuations may contribute to the vapor flux into a building (Robinson and Sextro, 1997).
57 Whether the subsurface vapor source is contaminated soil or groundwater, soil gas concentrations emanating from
a subsurface source generally attenuate, or decrease, as the vapors move from the source through the soil and into
indoor air due to diffusion and advection and are subject to any degradation. If and w hen soil vapor monitoring data at
a given site are not consistent with this trend, the possible existence of a preferential migration route(s) warrants
consideration. Sewer lines also warrant consideration as potential sources of vapors, as well as conduits for
preferential (e.g., unattenuated) transport of vapors towards buildings. Preferential migration routes are discussed
further throughout this Technical Guide, including in Sections 5.4, 6.3.2, and 6.5.2.
58 Low-permeability layer(s) overlying contaminated groundwater (i.e., "aquicludes") can, likewise, impede the flux of
vapors from the contaminated plume to the vadose zone. The aquiclude shown at the base of Figure 2-1 would not
impede the flux of vapors from the contaminated plume to the vadose zone, however, because the aquiclude is below
both. The aquiclude would impede vapor flux from any additional contaminated plume located below it.
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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Some chemicals (e.g., benzene, methane, and other petroleum hydrocarbons; vinyl
chloride (Patterson et al. 2013) and some otherchlorinated hydrocarbons) may
experience reductions in their soil gas concentrations due to biodegradation in the
vadose zone under certain conditions. Depending upon the potential for oxygen to
migrate into the subsurface and underneath buildings from the ambient air,
biodegradation may be anaerobic or aerobic. The rate of biodegradation in situ will be
chemical-specific (i.e., chemicals have different degradation rates even within a similar
microbial environment), will be site-specific (i.e., the microbial environment will depend
upon soil moisture, nutrient and oxygen levels, and the chemical mixture,59 among other
factors (Holden and Fierer 2005)), and may be location-specific (i.e., the microbial
environment can change overtime and space due to variations in soil moisture,60
nutrient and oxygen61 levels). In some cases, biodegradation in the vadose zone can
impede vapor migration significantly.
Demonstrating the extent, if any, to which these processes act as a barrier to vapor transport at
specific sites may entail intensive testing or investigative methods that are very different from
the sampling and analysis techniques for indoorair and soil gas (see, for example, Sections
6.3.2 and 6.4). Where and when it occurs, biodegradation may result in the formation of by-
products that are potentially hazardous (e.g., methane from ethanol, vinyl chloride from PCE or
TCE).
The distribution and magnitude of soil gas concentrations immediately beneath a building are
expected to reflect the interplay between vapor transport toward the building (via diffusion and
advection) in the vadose zone and vapor withdrawal due to soil gas entry into the building (in
the case where the building is under-pressurized), which may be spatially and temporally
variable (Section 2.3). Likewise, soil vapor may become contaminated as a result of over-
pressurized buildings forcing contaminated indoor air through openings in the foundation into
nearby soil.
2.3 Openings and Driving Forces for Soil Gas Entry into Buildings
Hazardous vapors in the vadose zone may eventually enter buildings as a component of a gas
by migrating through cracks, seams, interstices, and gaps in basement floors, walls, or
foundations ("adventitious openings") or through intentional openings, such as perforations due
to utility conduits and sump pits. Figure 2-2 illustrates some of these types of openings.
Buildings can be expected to vary, even within a single community, in the amount of opening
59 For example, aerobic biodegradation of benzene may be impeded by the presence of methane, due to competition
for oxygen by methane-oxidizing ("methanotrophic") bacteria, depending upon site-specific conditions (Ma et al.
2012).
Moisture plays a particularly important role for microorganisms in the vadose zone. Mcrobial growth and activity
can decrease rapidly with depth, coincident with the soil moisture profile, and increase again in the capillary fringe
(Holden and Fierer 2005).
61 Site-specific infrastructure and soil conditions, climate, and other factors will determine the extent to which oxygen
levels underneath a building will be different compared to locations outside the building footprint. In addition to
buildings, surf ace covers, such as asphalt or concrete, can impede oxygenation of the vadose zone, relative to the
case wherethe ground surface is in contact with the atmosphere, all other factors being equal.
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Assessing and Mitigating the Vapor Intrusion Pathway from
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CONCRETE SLAB
/ \ /
CRACKS OR \ \
HOLES EXPANSION
EXPANSION
JOINTS
CRAWL SPACE OR BASEMENT
WITH DIRT FLOOR
MOBILE HOME
BASEMENT WITH CONCRETE FLOOR
CRACKS &
OPENINGS
IN CINDER
BLOCKS
CINDER
BLOCKS
FLOOR-^
WALL
JOINTS
MORTAR
JOINTS
7 -
CRACKS A.
HOLES
A
\
SUMP
DRAIN
TILE PIPE
nuc nrc
Figure 2-2 Illustration of Potential Openings in Various Building Types
Note: Blue arrows represent soil gas migration or entry. Source: EPA (2008)
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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area available for soil gas entry; for example, buildings with deteriorating basements and
foundations or dirt floors are more susceptible to soil gas entry.
As mentioned in Section 2.2, advection in the vadosezone can arise in the vicinity of buildings
whenever there is a difference between the air pressure within a building and the subsurface
environment. The air pressure within a building can be lower than in the subsurface due to:
Temperature differences between indoor and subsurface locations (e.g., the winter-time
"stack effect," when buildings are commonly heated, leading to convection cells driven
by heated air that rises to upper levels and leaks through roofs and upper-floor
windows).
The operation of mechanical devices, such as exhaust fans for ventilation, air
conditioners, and clothes dryers, with vents to the outdoors.
The operation of fireplaces that vent combustion (exhaust) gases to the outdoors.
The operation of furnaces in basements of centrally heated buildings, which can
incrementally depressurize the basement (EPA 1993a).
Wind load on the building walls.
62
Even small pressure differences may cause advective flow of soil gas into or out of the building
through pores, cracks, or openings in the building floor or basement walls63 or gas present in
drain lines, sumps, and sewer lines that do not have adequate vapor traps.64
62 The wind effect is caused by differences in building pressure on a building's surf aces. The outdoor air pressure will
be higher on the windward side of the building, than on the leeward side, as ambient air flows around the building.
The net effect of this pressure difference will vary from building to building, depending upon the location of the
primary openings for soil gas entry (and the primary opening for air infiltration through the building envelope -see
Section 2.4) (EPA 1993a, Section 2.3 therein).
As a result of the construction of foundation walls and floor slabs, a perimeter crack(i.e., space between the floor
slab and walls) may be created and serve as an entry location for soil vapors. This perimeter crack is often obscured
by wall coverings, and may not be accessible for inspection or direct testing. Vapors have been observed to migrate
through what appears to be intact concrete floors and walls, which may, in fact, have small unobserved fractures or
porous areas from improper curing. In addition, conduits may be present that facilitate soil gas entry into buildings.
These conduits may include utility (e.g., sewer, water, or electrical) penetrations and floor drains
64 Where sewers or other conduits contain volatile contaminants, lateral lines connecting buildings to these conduits
may facilitate vapor intrusion into indoor air. Although floor drains are designed to allow water to drain away from the
building, they are usually not designed or constructed to eliminate gas entry. At a test house in Indianapolis, elevated
levels of PCE and chloroform were found in gas in a laundry drain line, which was suspected of serving as a source
of vaporsfound in indoor air (EPA 2012f). Although building construction codes and toilet designs are intended to
prevent sewer gas from entering homes, inadequate maintenance (e.g., plumbing fixture seals) can result in loss of
the intended protection. Pennell et al (2013), for example, found sewer gas entry to be a significant source of PCE in
indoor air at a home in Massachusetts. In addition, sewer gas was a suspected source of benzene in indoor air in
many buildings near a gasoline spill site in Hazleton, Pennsylvania
(www.epa.aov/rea3hwmd/npl/PA0001409671.htm). w here sew erventtraps were subsequently installed to mitigate
intrusion of gasoline vapors into homes.
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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To date, most analytical and computational models of vapor intrusion have been predicated on
the assumption that residences and other small buildings experience a constant under-
pressurization (i.e., lower pressure in the building than in the subsurface), which fosters vapor
intrusion. Whereas this assumption facilitates analyses and may be reasonable for some
purposes (see, for example, Section 2.5), it is highly idealized. To illustrate: fluctuations in
subslab-to-building pressure difference (and, hence, soil gas entry rates) overtime can be
reasonably expected due to:
diurnal (daily) and seasonal changes in the temperature of ambient air, whereas indoor
temperatures may be more stable, particularly during periods when mechanical heating
and cooling systems are in use;
changes in ambient air pressure;
non-instantaneous response (i.e., lag or delay in response) of subsurface soil gas to
changes in ambient air pressure (EPA 1993a, Section 2.3 therein), particularly where
low-permeability soil is in direct contact with a building foundation (e.g., basement)
below the ground surface;65
changes in wind direction and speed; and
intermittent operation of mechanical ventilation systems and combustion devices that
vent exhaust gases to the outside.
Theoretically, these processes and variables suggest that soil gas entry rates can be expected:
to vary over different time scales (e.g..within an day, and between seasons);
to differ geographically due to differences in ambient air temperature, pressure, wind,
and building conditions (e.g., leakage area and its distribution over the building
envelope); and
to be discontinuous over some time periods.
On the other hand, where granular fill is present underneath a building, there is potential for preferential soil gas
flow through the fill, especially in locations where the gas permeability of the surrounding soil is low. Where granular
materials have differentially settled, air voids (also highly permeable to soil gas flow) may form beneath the
foundation. Utility penetrations and other conduits may be connected to the granular fill, accentuating the potential
pathway for soil gas entry into a building. Adding to the complexity, pressure differentials caused by windflows
conceivably could create a cross-flow through granular fill underneath the foundation, which may episodically dilute
vapor concentrations (and oxygenate soil gas) in the building vicinity.
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2.4 Air Exchange and Mixing
Air exchange refers to the flows into and out of a building, which are generally in balance, and is
composed of three processes:
1) infiltrationair leakage through random cracks, interstices, and other unintentional
openings in the building envelope;
2) natural ventilationairflows through open windows, doors, and otherdesigned
(intentional) openings in the building envelope; and
3) mechanical ventilationair movement controlled and driven by fans.
For the vapor intrusion exposure scenario, air exchange by each of these processes will
generally tend to mitigate the effects of vapor intrusion (i.e., reduce indoor air concentrations)
via dilution, while air inflows will also transport indoors any vapor-forming chemicals in ambient
air (see Section 2.7).66
The air exchange rate is conventionally defined as the ratio of the airflow rate (e.g., cubic
meters per second) to the building volume (e.g., cubic meters) and is generally expressed in
terms of exchanges per hour (i.e., overall units of hour1). Valuesfor residential air exchange
rates are typically on the order of approximately 0.18 to 1.26 air changes per hour (ACH) (EPA
2011b, see Table 19-24 therein, 10th and 90th percentiles).67'68 Values for non-residential
buildings are highly-dependent upon building use and can range widely (on the orderof
approximately 0.3 to 4.1 ACH) (EPA 2011b, see Table 19-27 therein, 10th and 90th percentiles).
The potential diluting effect of air exchange arises when ambient air has negligible presence of the volatile
chemicals found in site-related contamination in the subsurface environment. In some situations, site-related
contamination has the potential to impact ambient air with the same vapor-forming chemicals that pose a threat from
vapor intrusion. For example, contamination of shallow soil or groundwater may release site-related vapor-forming
chemicals to ambient air. In such situations, air exchange would contribute to the presence of site-related
contamination in indoor air, rather than only dilute any impacts from vapor intrusion.
EPA's Office of Research and Development evaluated eight studies of air exchange rate for residential buildings
and selected a 1995 EPA study as the basis for recommending values for risk assessment (EPA 2011b, Table 19-
24). The key study analyzed almost 3,000 time-averaged measurements of exchange rate in occupied homes in the
United States, which were generally obtained using a tracer-release method. Median values ranged from 0.35 hour"
in the mid-western region to 0.49 hour" in the northeast and southern regions. Tenth percentile values ranged from
0.16 hour"1 in the mid-western and southern regions of the U.S. to 0.23 hour"1 in the northeast region. Regional
differences in exchange rate reflect differences in weather(e.g., temperature and windspeed), prevailing building
conditions (e.g., house'leakiness'), and the time periods (e.g., season) in which measurements weremade.
68 EPA's Off ice of Research and Development (EPA 2011b, Section 19.5.1.2.7) also summarized a study that
conducted approximately 500 indoor-outdoor air exchange rate (AER) calculations based on residences in three
urban locations (metropolitan Elizabeth, NJ; Houston, TX; and Los Angeles, CA). This study highlights how climate
and season can influence air exchange rate. In Texas, the measured AERs were lower in the summer cooling season
(median =0.37 ACH) than in the winter heating season (median = 0.63 ACH), likely because windows were closed
while air conditioners werein use. The measured AERs in California were higher in summer (median = 1.13 ACH)
than in winter (median = 0.61 ACH), because summers in Los Angeles County are less humid than NJ or TX and
residents are more likely to utilize natural ventilation through open windows andscreened doors. In New Jersey, air
exchange rates in the heating and cooling seasons were similar.
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To date, most analytical and computational models of vapor intrusion have been predicated on
the assumption that residences and other small buildings are well mixed spaces throughout
which concentrations of vapor-forming chemicals are uniform. Whereas this assumption
facilitates analyses and may be reasonable for some purposes (see Section 2.5), it is highly
idealized. To illustrate: airflowwithin a building (i.e., inter-zonal airflow) can be impeded by
doors, walls, and other partitions that separate rooms and other building areas. Whereas
airflows within a building can be facilitated by mechanical means, spatial variation of
temperature and humidity suggest that air mixing is not necessarily complete even in buildings
that benefit from centralized systems for heating, air condition, and ventilation. Furthermore,
many residences do not have such mechanical systems. Therefore, buildings subject to vapor
intrusion may exhibit differences in concentration of vapor-forming chemicals among building
areas (e.g., rooms) as a result of the differential proximity to openings for soil gas entry (see
Section 2.3) and openings for air leakage and ventilation and the magnitude and balance of
inter-zonal airflows. For example, rooms with perforations through the foundation (e.g.,
bathrooms or utility rooms) may have greater concentrations of vapor-forming chemicals in air
compared to rooms that do not. Generally, basements can reasonably be expected to exhibit
greater concentrations of vapor-forming chemicals than upper occupied levels.
Buildings constructed overa crawl space with a dirt floor may benefit from the dilution of soil gas
by any ventilation of crawl space air, but would not have the impedance to vapor intrusion that
concrete slabs can provide. Trailers enclosed at the bottom by a skirt are expected to have
greater potential for vapor intrusion than would non-enclosed trailers. Wind movement between
the ground surface and the bottom of the non-enclosed trailer would tend to minimize vapor
buildup and associated potential for vapor flux into the building. Similarly, the existence of
underground parking for a multi-story building (orother modifications to the foundation that
enhance subsurface ventilation) would tend to minimize the potential for vapor intrusion.
2.5 Conceptual Model Scenarios
Based upon the foregoing conceptual model, numerous factors can influence the potential
indoor air concentration arising from vapor intrusion. Some of these significant factors are
illustrated in Figure 2-3.
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Vadose Vadose Vadose Building
apor zone Zone Zone Bio- Found-
lOi i roo
Geology Hydrology chemistry ation
Greater
Vapor
Intrusion
Potential
Less
Vapor
Intrusion
'Potential
High Source
Cone.,
Highly
Volatile
Chemicals
See
Sections 2. 1
and 6. 3.1
Low Source
Cone., Less
Volatile
Chemicals
Vertically
Fractured or
Coarse-
Grained,
Vertically
Uniform
Media
See
Sections 2. 2
and 6, 3.2
Horizontal
and
Laterally
Extensive
Fine-
G rained
Layers
Low
Moisture
Content in
Vadose
Zone,
Shallow
Water
Table,
Large
Water Table
Fluctuations
See Sections
2.1, 2. 2 and
6.3.2
High
Moisture
Content in
Vadose
Zone, Deep
Water Table,
Think*
i Mluiv
Capillary
Fringe
Unfavorable
for
Complete
Degradation
or Non-
Degradable
Chemicals
See
Sections 2.2
and 6. 3.2
Favorable
for
Complete
Degradation
and
Degradable
Chemicals
Cracked
Slab,
Partial
Slabs,
Sumps or
Drains
See
Sections
2.3, 6.3.3,
and 6.4.1
Intact ,
Extensive,
and
Thicker
Slab
Figure 2-3 Some Factors That Affect Vapor Intrusion
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The document Conceptual Model Scenarios forthe Vapor Intrusion Pathway (EPA 2012b)
provides simplified simulation examples69 to illustrate graphically how several of the subsurface
and building-specific factors work together to determine the distribution of volatile contaminants
in the subsurface and the indoorair concentration relative to a source concentration. The
conceptual model scenarios document offers insights into the factors influencing the vapor
intrusion pathway. It provides a theoretical framework with which to draw inferences about and
better understand the complex vapor fate and transport conditions typically encountered at
actual, non-idealized contaminated sites. The following general observations can be made from
these simplified simulation examples, and may be useful when considering the vapor intrusion
pathway at a particular site:
The horizontal and vertical distance over which vapors may migrate in the subsurface
depends on the source concentration, source depth, soil matrix properties (e.g., porosity
and moisture content), and time since the contaminant release to the environment
occurred. Months or years of volatilization and vapor migration may be required to fully
develop vapor distributions in the vadose zone at sites with deep vapor sources or with
impedances to vapor migration arising from hydrologic or geologic conditions.
Vapor concentrations, including oxygen, in the vadose zone (i.e., soil gas
concentrations) may not be uniform in sub-slab soil gas or in soil gas at similar depths
exterior to the building of interest. Therefore, soil gas concentrations at exterior locations
(i.e., outside a building's footprint) may be substantially differentfrom the concentration
underneath the building (e.g., the sub-slab concentration), depending on site-spedfic
conditions and the location and depth of the exterior soil gas sample.
Simulations assuming an idealized, constructed ground cover suggest that shallow soil
gas concentrations can be greater under low-permeability ground covers (e.g., asphalt)
than under soil open to the atmosphere.
The soil gas distribution beneath a building is not the only factor that determines the
indoor air concentration. The indoor air concentration is also influenced by building
conditions, including the presence of openings (e.g., cracks, utility penetrations) in the
foundation, building pressurization, and the air exchange rate.
Advective flow into buildings, which is presumed to occur predominantly near cracks and
openings in the foundation slab, may affect the distribution of vapor-forming chemicals
directly beneath the structure. Heterogeneities in the permeability of geologic materials
and backfill, along with wind effects and building and atmospheric pressure temporal
variation, may also contribute to the spatial and temporal variability of vapor
concentrations in sub-slab soil gas and indoor air.
Two important simplifications are the assumptions of constant values forthe driving force for vapor intrusion (i.e.,
subslab to indoor air pressure difference) and air exchange rate, whereas time-variable values are reasonably
expected as a result of changing weather and other conditions (as summarized in Sections 2.3 and 2.4, respectively).
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Subsurface heterogeneities in site geology, such as layering and moisture content, can
influence the extent and rate of vapor migration from a contaminant source towards
overlying or adjacent buildings.
The soil gas distribution of aerobically biodegradable chemicals (e.g., BTEX) can be
significantly different than that of otherchemicals that are not biodegradable (i.e., are
recalcitrant) in similar settings. Specifically, the soil gas concentrations of aerobically
biodegradable chemicals exhibit greater attenuation than those of recalcitrant chemicals
when the subsurface availability of oxygen is adequate.
2.6 Variability in Exposure Levels
Given the foregoing conceptual model of vapor intrusion and summary of modeled scenarios
(EPA 2012b), the degree to which vapor intrusion is a pathway of concern can vary widely from
site to site and from building to building within a site. Adding to the complexity, theoretical
considerations (i.e., soil gas entry rates, air exchange rates, interior compartmentalization and
inter-zonal airflows) suggest that indoor air concentrations arising from vapor intrusion can be
expected to vary overtime and within a building. Field observations and measurements
demonstrate that indoorair concentrations can exhibit significant temporal variation within a day
and between days and seasons in an individual residential building (EPA 2012a; Holton et al.
201 Sab).
2.7 Consideration of Indoor and Outdoor Sources ofVOCs
Indoor air in many buildings will contain detectable levels of a number of vapor-forming
chemicals whether or not the building overlies a subsurface source of vapors (EPA 2011 a),
because indoor air can be impacted by a variety of indoor and outdoor sources. Indoor sources
of volatile contaminants include the use and storage of consumer products (e.g., cleaners, air
fresheners, aerosols, mothballs, scented candles, insect repellants, orotherhousehold
products), combustion processes (e.g., smoking, cooking, and home heating), occupant
activities (e.g., craft hobbies, home improvements, automotive repairs), and releases from
interior building materials (e.g., carpets, insulation, paint, and wood-finishing products). Outdoor
sources of volatile chemicals may arise due to releases from nearby sources such as industrial
facilities, vehicles, yard maintenance equipment, fuel storage tanks, and paint or pesticide
applications; regional sources such as air emissions from regional industry, vehicle exhaust,
agricultural activities, and fires; or global sources, such as distant air emissions. The outdoor air
surrounding a building is referred to as "ambient air" throughout this Technical Guide.
The contribution of indoor and outdoorsources of vapors (or both) to indoor air concentrations,
which do not arise from site-related contamination,70 is referred to as "background" throughout
this Technical Guide (see, for example, Sections 2.7, 6.3.5, and 7.4.2 and the Glossary). In
70 In some situations, site-related contamination has the potential to impact indoor or ambient air (EPA 1993c) with
the same vapor-forming chemicals that pose a threat from vapor intrusion. For example, contaminated groundwater
in building sumps or intruding into the building via groundwater seepage could provide an indoor source of site-
related contamination. Contamination of shallow soil or groundwater may also release site-related vapor-forming
chemicals to ambient air. In such situations, neither of these sources of indoor air contamination would be considered
'background.'
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some buildings, "background" sources by themselves can cause building occupants and visitors
to experience significant exposures to vapor-forming chemicals.
In contrast to "background" concentrations in soil arising from naturally occurring minerals,
"background" concentrations in indoor air often are not uniform in time. For example,
concentrations of vapor-forming chemicals in ambient air may exhibit temporal variation over
several time scales (e.g., daily, seasonal, longer term) and spatial differences across urban,
suburban, and rural land use areas, reflecting differences in emission sources and rates and
environmental factors that transport, disperse, and remove these pollutants (Jia et al. 2012 and
citations therein). Concentrations of vapor-forming chemicals arising in indoor air in residential
buildings due to indoor sources have been observed to depend upon season and otherfactors.
Available studies suggest complex(e.g., patchy) spatial patterns in exposure concentration,
which has led some researchers to refer to "microplumes" in the indoor air environment
(McBride et al., 1999 and citations therein).
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3.0 OVERVIEW OF THIS VAPOR INTRUSION TECHNICAL GUIDE
This section provides an overview of this Technical Guide and the general recommended
framework for vapor intrusion assessment and response action, which is illustrated in Figure 3-
1. This section opens with a description of subsurface contaminants that have the greatest
potential to pose a health concern via vapor intrusion, based upon their volatility and potential
hazards.
3.1 Contaminants of Potential Concern
Several physicochemical criteria may be considered for defining and screening for volatility.71
For purposes of this Technical Guide, a chemical generally is considered to be "volatile" if:
1) Vapor pressure is greaterthan 1 millimeter of mercury (mm Hg), or
2) Henry's law constant (ratio of a chemical's vapor pressure in air to its solubility in water)
is greater than 10"5 atmosphere-meter cubed per mole (atm m3 mol"1) (EPA 1991 b,
Section 3.1.1; EPA2002c, AppendixD).
Various other criteria may be considered for identifying when volatile chemicals are present at
levels of potential health concern. For purposes of this Technical Guide, a volatile chemical
generally is considered to be "potentially toxic" via vapor intrusion if:
1) the vapor concentration of the pure component exceeds the indoorair target risk level,
when the subsurface vaporsource is in soil, or
2) the saturated vapor concentration exceeds the target indoor air risk level, when the
subsurface vaporsource is in groundwater.
Each of the chemicals with one or more toxicity values used to derive Regional Screening
Levels (http://www.epa.aov/rea3hwmd/risk/human/rb-
concentration table/Generic Tables/index htm) were evaluated for volatility and toxicity,
according to the foregoing recommended criteria. These criteria do not include a consideration
of whether these chemicals are regulated pursuant to CERCLA, as amended, or RCRA, as
amended. The universe of chemicals evaluated and the results of the evaluation are provided in
EPA's on-line VISL Calculator (EPA 2015a), which is described further in Sections 1.4.1 and
6.5.2 of this Technical Guide.
Chemicals which satisfy the foregoing screening criteria forvolatility and toxicity are designated
as "vapor-forming chemicals" for purposes of this Technical Guide. In addition:
In chemistry and physics, volatility refers to the tendency of a substance to form vapors, which are molecules in a
gaseous state, and escape from a liquid or solid. Volatility is directly related to a substance's vapor pressure and
Henry's law constant. EPA (1991b) also cites molecular weight as a necessary criterion for assessing volatility.
Molecular weight is not retained for this Technical Guide as a volatility criterion, because it is a relatively weak
predictor of volatility.
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cis-1,2-dichloroethylene, a volatile chemical that lacks sufficient toxicity information (to
apply the toxicity criteria above), is identified as a vapor-forming chemical due to its
potential use as an indicator of vapor intrusion when it is present as a subsurface
contaminant;72
methane is identified as a vapor-forming chemical due to its potential to pose an
explosion hazard and to be formed via anaerobic biodegradation processes in the
subsurface environment;73and
radon is identified as a vapor-forming chemical when it arises from uranium- or radium-
bearing solid wastes in the subsurface.74
Chemicals that meet these recommended screening criteria are referred hereafter in this
Technical Guide as "vapor-forming chemicals." EPA recommends that these chemicals be
evaluated during vapor intrusion assessments, when they are present as subsurface
contaminants due to a site-related release(s). EPA recommends that chemical analyses be
limited to those vapor-forming chemicals known or reasonably expected to be present in the
subsurface environment due to a site-related release(s). The list of vapor-forming substances
warranting consideration for potential vapor intrusion may be modified in the future.75
3.2 Vapor Intrusion Assessment
The approach for assessing vapor intrusion will vary from site to site, due to site-specific factors.
For example, the information available for evaluating vapor intrusion potential will vary
depending upon when vapor intrusion is first considered during a site's investigation-and-
cleanup life cycle. Many sites can be evaluated for potential vapor intrusion during the normal
course of an initial site assessment. Examples include brownfield sites that are intended for
redevelopment and buildings where chemical odors have been reported. The data available for
evaluating vapor intrusion may be very limited at the outset for these situations. At the other end
of the investigation and cleanup life cycle, certain sites with long- term cleanups underway for
contaminated groundwatermay be evaluated for vapor intrusion during periodic reviews, if any,
72 EPA (2011 a) and DoN (2011 a) report that cis-1,2-dichloroethylene (cis-1,2-DCE) is "rarely detected in background
indoor air." When they are subsurface contaminants, volatile chemicals, such as cis-1,2-DCE, that are rarely or never
present in indoor sources can be inferred to arise in indoor air via vapor intrusion "withoutf urther explanation" (DoN
2011a). Brenner (2010), for example, employed this principle to identify buildings susceptible to vapor intrusion and to
diagnose the relative contributions of vapor intrusion and infiltration to indoor air concentrations.
73 As noted previously, methane in soil gas may produce twoother undesirable consequences: (1) it can exacerbate
migration and intrusion of other vapors if it is generated at rates sufficient to foster advective flow of soil gas (see
Section 2.3); and (2) its biodegradation in the vadosezone can reduce the oxygen available for biodegradation of
other hydrocarbons (Ma etal. 2012).
Radon emanating from natural geological materials may also affect indoor air quality in occupied buildings, but is
not a subject of this Technical Guide. For more information about radon emanating from natural geological materials,
see: http://www.epa.aov/radon/index.html.
75 For example, inhalation toxicity values for additional volatile chemicals may become available in the future.
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o
Q.
TJ
E
ro
4-ป
E
OJ
8
BO
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01
E
O
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o
u
Preliminary Analysis (See Section 5)
Assemble, Evaluate, and Review Available Information (Sections 5.1 - 5.3, Appendix A)
Develop Initial Conceptual Site Model (Section 5.4)
Does available
information indicate
conditions that warrant
prompt response action^
(Section 5.2)
Does available
information indicate a
potential for vapor-
forming chemicals to be
present in the
subsurface?
(Sections 3.1
and 5.3)
No Further Vapor Intrusion Assessment
Needed if there are sufficient data of
appropropriate quality to support
decisions for current and/or future
conditions.
Are
institutional controls in
place to prevent
development or ensure
additional VI
investigation for new
building(s)?
Does available
information indicate
actual or potential future
presence of buildings
nearby? (Section 5.3)
Plan and Conduct Detailed Vapor Intrusion
Investigation and Evaluate Data (See Figure 6-1)
Evaluate response actions, including
institutional controls (ICs), to mitigate
exposure in current and future buildings.
When infrastructure conditions change,
EPA recommends a vapor intrusion
investigation or pre-emptive mitigation be
conducted (Sections 3.3 and 7.8).
intrusion concern
No Further Vapor Intrusion Assessment Needed if there are sufficient data of
appropriate quality to support decisions for current and/or future conditions.
Figure 3-1 Overview of Recommended Framework forVapor Intrusion Assessment and Response
Action
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of remedy performance and groundwater monitoring data.76 In such situations, detailed
information about the nature and extent of subsurface contamination and the relevant
hydrogeologic conditions may already exist. In addition, there are different scenarios for vapor
intrusion (EPA2012b), depending on characteristics of the source (e.g., types, chemicals of
concern, mass, distribution, and distance from building(s)), subsurface conditions and vapor
migration routes (e.g., soil types and layering, existence of preferential migration routes dueto
geology or infrastructure, and existence of any impediments to vapor migration), building
susceptibility (e.g., age, design, construction, condition), lifestyle factors (e.g., keeping windows
open or closed), and regional climate. For these reasons, every site (and every building) will not
warrant the same approach to or intensity of assessmentfor vapor intrusion. This Technical
Guide, therefore, recommends a framework for planning and conducting vapor intrusion
investigations, rather than a prescriptive step-by-step approach to be applied at each and every
site.
Broadly speaking, two general levels of vapor intrusion assessments can be distinguished:
1) A preliminary analysis utilizes available and readily ascertainable information to develop
an initial understanding of the potential for human health risk that are or may be posed
by vapor intrusion, which would typically be performed as part of an initial site
assessment. The recommended information, approaches, and practices for conducting a
preliminary analysis and its potential outcomes are described in Section 5.0.
2) A detailed investigation is generally recommended when the preliminary analysis
indicates that subsurface contamination with vapor-forming chemicals may be present
underlying or near buildings. A detailed investigation of the vapor intrusion pathway is
typically performed as part of the site investigation stage. The recommended
approaches and practicesfor conducting detailed vapor intrusion investigations are
described in Section 6.0.
Considerable information, primarily empirical, has been generated regarding evaluation of the
vapor intrusion pathway since the pathway emerged as a national issue in the late 1990s and
especially in the past ten years. Broadly speaking, this information demonstrates that the vapor
intrusion pathway can be complex (The conceptual model of vapor intrusion provided in Section
2.0 identifies many of the factors, variables, and conditions that warrant consideration on a site-
specific basis.) Asa result, current practice suggests that the vapor intrusion pathway generally
be assessed using multiple lines of evidence.77
Specific conclusions that EPA recommends be based upon multiple lines of evidence include:
76 These situations can arise, for example, if the groundwater remedy was selected in the 1980s (long before vapor
intrusion became recognized as a potentially significant exposure pathway), or if supplemental groundwater data
indicate that the plume is migrating toward new inhabited areas.
77 As discussed further in Section 7.2, confidence in the assessment and risk management decisions is expected to
be higher when multiple independent lines of appropriate site- or building-specific evidence from, for example,
multiple types samples of environmental media (e.g., groundwater, soil-gas, sub-slab vapor, crawlspace, and indoor
air) and/or other data come together to provide mutually supporting evidence fora common understanding of the site
conditions/scenarios and the potential for vapor intrusion (EPA 2010b) (i.e., the various lines of evidence are in
agreement with each other).
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The subsurface vapor source(s) at a specific site has (or, alternatively, does not have)
the potential to pose an unacceptable vapor intrusion exposure under current or
reasonably expected future conditions, due to its strength (e.g., concentration and mass
of vapor-forming chemicals) and proximity relative to one or more existing buildings or a
building that may be constructed in the future.
The vapor intrusion pathway is complete for one or more buildings under current
conditions or is potentially complete under reasonably expected future conditions.
The vapor intrusion pathway is incomplete for one or more buildings near a subsurface
source of vapor-forming chemicals, due to geologic, hydrologic, and/or biochemical (e.g.,
biodegradation) processes that provide substantial and persistent attenuation of vapors
extending laterally over large distances relative to the footprint of the building(s) and the
extent of the vapor source.
Indoor air concentrations attributable to vapor intrusion pose (or, alternatively, are
unlikely to pose) an unacceptable human health risk in one or more existing buildings
under current or reasonably expected future conditions.
Indoor air concentrations measured in one or more buildings can (or alternatively,
cannot) be reasonably attributed to indoor or ambient air sources (i.e., background).
Multiple lines of evidence are particularly important for supporting "no-further-action" decisions
regarding the vapor intrusion pathway (e.g., pathway incomplete determinations) to reduce the
chance of reaching a false-negative conclusion (i.e., concluding vapor intrusion does not pose
unacceptable human health risk, when it actually poses an unacceptable human health risk).
Collecting and weighing multiple lines of evidence can also help avoid reaching a false-positive
conclusion (i.e., concluding vapor intrusion poses an unacceptable human health risk, when it
does not.
In summary, EPA recommends that site assessors generally collect and weigh multiple lines of
evidence, including qualitative information, to support decision-making regarding the vapor
intrusion pathway. Lines of appropriate evidence to evaluate the vapor intrusion pathway were
identified in Section 2 and are discussed further in Sections 5 through 7.
As noted in Section 1.3, Figure 3-1, and the preceding discussion of lines of evidence, EPA
recommends that site assessors consider reasonably expected future conditions, in addition to
current conditions, when reaching condusions aboutthe vapor intrusion pathway.78 Forthis
reason, this Technical Guide includes recommendations for evaluating whether subsurface
vapor sources that remain have the potential to pose unacceptable human health risks in the
future if current conditions were to change. Forexample:
Section 6.3.3 recommends that site assessors consider investigating vapor intrusion in
non-residential buildings underconditions when the heating, ventilation, and air-
conditioning system is not operating; and
"Both current and reasonably likely future risks need to be considered in order to demonstrate that a site does not
present an unacceptable risk to human health and the environment." (EPA 1991a).
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Section 7.3 identifies some factors and processes that can make the characteristics of
the vadose zone (e.g., soil moisture) transitory.
EPA also recommends that vapor intrusion be evaluated for reasonably expected future land
use conditions, including new building construction and new uses and occupants for any
uninhabited buildings.
3.3 Building Mitigation and Subsurface Remediation
The NCR expresses the preference for response actions that eliminate or substantially reduce
the level of contamination in the source medium to acceptable levels, thereby achieving a
permanent remedy. In the case of vapor intrusion, such a response action would generally entail
eliminating or substantially redudng the level of vapor-forming chemicals in the subsurface
(e.g., in contaminated groundwater, soil, and/or sewer lines) via treatment or removal (i.e.,
"remediation"). Section 8 discusses source remediation to achieve a permanent remedy and
associated institutional controls (ICs) and monitoring for vapor intrusion mitigation, including
criteria for their termination.
Comprehensive remediation79 of the subsurface environment often occurs over a prolonged
period (e.g., several years) to attain cleanup levels. In the interim, problems of unacceptable
vapor intrusion are often addressed by also installing engineered exposure controls to reduce or
eliminate vapor intrusion in buildings (i.e., "mitigate" vapor intrusion) or reduce indoor air
exposure levels. Engineered exposure controls can generally be deployed and generally
become effective relatively quickly. They can be considered as "interim" or "early" response
actions, which are also authorized by the NCR (Section 1.2), as necessary and appropriate to
promptly reduce threats to human health. Section 8 summarizes technical information about
specific exposure controls and provides information about theiroperation, maintenance and
monitoring and associated ICs, including criteria for theirtermination.
Functionally, engineered exposure controls can be categorized into two basic strategies:
Those that seek to prevent or reduce vapor entry into a building (e.g., active
depressurization technologies). These methods are more commonly implemented when
response actions are needed.80
Those that seek to reduce or eliminate vapors that have entered into a building (e.g.,
indoor air treatment, ventilation).
79 For purposes of this Technical Guide, "remediation" is intended to apply to interim and final cleanups, whether
conducted pursuant to RCRA corrective action, the CERCLA removal or remedial programs, or using EPA brownfield
grant funds with oversight by state and tribal response programs. In addition to permanent remedies for subsurface
vapor sources, site remediation may also entail implementation of ICs and construction and operation of engineered
systems to reduce risks to human health and the environment posed by environmental pathways other than vapor
intrusion. Although this Technical Guide is intended for use at any site subject to federal statutes, regulations, and
rules, it is not intended to alter existing requirements, guidance, or practices in OSWER's programs about
development, selection, or documentation of final "remediation" plans (addressing subsurface vapor sources, for
example) - see, for example, Sections 7.6 and 7.7.
Ivltigation methods that prevent or reduce vapor entry into a building from subsurface vapor sources would
generally also be expected to reduce radon entry.
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Neither strategy entails reducing the level of vapor-forming contamination in the subsurface
source medium.81
As reflected in the foregoing conceptual model of vapor intrusion (Section 2.0), entry of the
vapors into a building may be prevented or reduced by any of several techniques, which have
the following objectives:
Remove or reverse the driving forces for vapor intrusion into the building (e.g., install
and operate an active depressurization technology to mitigate vapor intrusion from
contaminated soil orgroundwater; establish over-pressurization within and throughout
the footprint of a nonresidential building).
Eliminate or minimize identified openings for vaporentry into the building (e.g., caulking,
grouting, or otherwise sealing all holes, cracks, sumps and otherfoundational openings
or creating a barrier between the soil and the building that blocks openings for entry of
soil gas into the building; install, repair, and/or maintain vapor traps in sewer and drain
lines).
Engineered exposure controls that entail mechanical systems and forces (e.g., sub-slab
depressurization or ventilation systems; building over-pressurization) are often referred to as
"active." Engineered exposure controls that do not involve mechanical operations (e.g., installing
a sub-slab barrier to chemical vapor entry) are often referred to as "passive." Many building
mitigation systems rely on both active and passive strategies.
Engineered exposure controls that seek to reduce or eliminate vapors that have entered into a
building can also be effective. In some instances, they can be implemented more readily than
engineered exposure controls that reduce or eliminate entry of the vapors into a building.
Typically, the simplest approach to limiting the concentration levels in occupied indoorspaces is
to increase building ventilation (i.e., increase the rate at which indoor air is replaced with
outdoor air), thereby diluting indoor air concentrations (see Section 2.4).82 Alternatively, vapor-
forming chemicals are removed from indoor air using an adsorbing material (such as activated
carbon) that can be either properly disposed of or recycled. Building mitigation methods that act
upon vapor-forming chemicals in indoor air (i.e., rely upon enhanced ventilation or treatment)
are generally capable of reducing background levels of chemicals, in addition to reducing indoor
levels of vapor-forming chemicals that intrude from subsurface sources.
81 Even when operated for prolonged periods, engineered exposure controls are considered 'interim' remedies for
purposes of this Technical Guide, because their implementation does not substitute for remediation of the subsurface
source(s) of vapor-forming chemicals. Engineered exposure controls may, nevertheless, become part of a final
cleanup plan.
82 It can be difficult to establish a ventilation rate that mitigates vapor intrusion and yields an environment conducive
to human occupancy (e.g., considering air temperature or moisture). In addition, ventilation may affectthe driving
forces for vapor intrusion. For example, mechanically exhausting air from the building will generally contribute to
building under-pressurization (see Section 2.3), which may result in increased intrusion of soil gas into the building,
which may offsetthe intended dilution effect of ventilation. On the other hand, introducing outdoor air at a rate slightly
greater than the exhaust rate can create over-pressurization, which opposes the primary driving force for vapor
intrusion.
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Selection of an interim response action from these options may depend upon building- and site-
specific factors (EPA 2008). For example, building-specific factors may include:
Use (e.g., single-family residential, multi-family residential, commercial, educational,
recreational, governmental, religious, industrial)
Type of foundation/basement (e.g., basements with concrete slab floors or dirt floors,
slab on grade, slab belowgrade) and other construction features
Type of heating/cooling/ventilation systems (e.g., some systems will tend to increase
pressure, whereas others will tend to decrease pressure, inside the building).
Each of these characteristics can influence the choice of mitigation methodology and, therefore,
they are commonly identified during building surveys during a site-specific vapor intrusion
investigation. Site-spedfic considerations may include the degree of risk or hazard being
addressed and whether the subsurface vapor source(s) is stable in extent and concentration or
is undergoing remediation.
Temporary relocation may warrant consideration in instances where explosion hazards are
present (see Section 7.5.1), which may pose an imminent and substantial danger to human
health and public welfare. Prompt response action may also be warranted where short-term or
acute exposures may pose unacceptable human health risk (see Section 7.5.2) that cannot be
addressed timely or feasibly by implementing engineered exposure controls. Section 8.2
discusses various prompt response actions for such situations, which may include temporary
relocation.
There may be situations where a party may wish to implement mitigation or control measures
for vapor intrusion, even though only limited lines of evidence or measurements may be
available to characterize the overall vapor intrusion pathway. For example, a party may be
aware that vapor intrusion has been documented at neighboring structures, where measures
are being implemented to mitigate the vapor intrusion pathway. A party may conclude there is a
reasonable basis to take action, but each building presents a fact-specific situation that calls for
its own individual judgment. Likewise, it may be appropriate and cost-effective to design, install,
operate, and monitor engineered exposure controls for individual buildings to mitigate vapor
intrusion in newly constructed buildings, or in buildings to be constructed in the future, that are
located in areas of vapor-forming subsurface contamination, rather than potentially allow vapor
intrusion to occur later and assess vapor intrusion after the fact. The term "preemptive
mitigation/early action" is used in this Technical Guide to describe these situations.83
The decision for preemptive mitigation/early action arises from precaution and from recognizing
that:
Installing engineered exposure controls in buildings is typically a cost-effective means of
protecting human health and normally can be implemented relatively quickly in many
buildings while subsurface contamination is being delineated or remediated.
The term 'preemptive' has been used to describe the use of various types of controls that can prevent vapor
intrusion from occurring prior to having fully demonstrated that unacceptable vapor intrusion currently exists in
specific buildings being considered (EPA 201 Oa).
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Conventional vapor intrusion investigations can be disruptive for building occupants
(residents, workers, etc.) and owners.
Comprehensive subsurface characterization and investigation of vapor intrusion can
entail prolonged study periods, during which time building occupants and owners and
others may have questions and concerns about human health risk that are or may be
posed via vapor intrusion.
Early action and interim action are allowed by federal environmental protection statutes,
regulations, and guidance, including CERCLA, as amended, and RCRA, as amended - see
Section 1.2 of this Technical Guide, for example. Other aspects of preemptive mitigation/early
action are also discussed in Section 7.8, including situations and criteria for decision-makers to
consider.
As noted in Figure 3-1, EPA recommends that risk managers consider reasonably expected
future conditions, in addition to current conditions, when making risk management decisions
about the vapor intrusion pathway. For this reason, this Technical Guide includes
recommendations for response actions at sites where subsurface vapor sources remain into the
future, but do not pose unacceptable human health risk under current conditions (e.g., no
building is present nearby). For example, institutional control s are generally recommended to
restrict land use and/or alert parties (e.g., prospective developers, owners, and municipalities) of
the presence of subsurface sources of vapor-forming chemicals at levels that pose a continuing
threat via vapor intrusion (see Sections 7.3,7.4, and 8.6). When infrastructure conditions
change above or near an area of known contamination with vapor-forming chemicals, EPA
recommends a vapor intrusion investigation or pre-emptive mitigation be conducted, particularly
if a building is constructed for human occupancy (see Section 8.2.3).
3.4 Community Outreach and Involvement
EPA is committed to transparency and upfront collaboration with community stakeholders
regarding land cleanup, emergency preparedness and response, and management of
hazardous chemicals and wastes. OSWER's Community Engagement Initiative (CEI), in
particular, is designed to enhance OSWER's and the Regional offices' engagement with local
communities and stakeholders (e.g., state and local governments, tribes, academia, private
industry, other federal agencies, and nonprofit organizations) to help them participate
meaningfully in government decisions regarding OSWER's nationwide programs.
Meaningful and sustained community outreach and engagement efforts are critical to the
implementation of work plans for site-specific vapor intrusion assessment and mitigation.
Because assessing the vapor intrusion pathway may involve sampling in a home or workplace,
as well as other temporary inconveniences (e.g., assisting in reducing indoorsources of
contaminants), individual, one-on-one communication with each property owner or renter
generally warrants consideration. Building-by-building contact and communication are
recommended as the most effective means of educating the community and obtaining access
needed to assess, mitigate, and monitor the vapor intrusion pathway. Personal contact is further
recommended to establish a good working relationship with each building owner or occupant
and to build trust. In many instances, local religious and cultural organizations, and other
community groups can be sought for assistance in reaching out to affected community
members.
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Vapor intrusion education and training are important components of meaningful community
outreach and engagement efforts. Informing stakeholders about the vapor intrusion pathway
and the cleanup process can help to build trustand can fostercommunity participation in the
overall assessment and risk management process.
Recognizing the importance of community outreach and engagement efforts, EPA staff are
highly encouraged to consult with colleagues experienced in community outreach and utilize
available EPA planning resources, including those discussed in Section 9, which provides
OSWER's community involvement planning guide for vapor intrusion projects. Like EPA, the
ITRC also recommends implementing a community outreach program that provides timely
information to concerned community members and property owners.
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4.0 CONSIDERATIONS FOR NONRESIDENTIAL BUILDINGS
The approach for investigating and, if necessary, mitigating vapor intrusion can vary from site to
site, and from building to building, due to site- and building-specific factors and circumstances,
including: the nature (e.g., mixture of vapor-forming chemicals and form), locations, and extent
of subsurface contamination; geologic, hydrologic, and biochemical factors in the vadose zone;
and the size, structural conditions and uses of buildings and background levels of vapor-forming
chemicals in the building. Information on 'background' contributions of site-related, vapor-
forming chemicals in indoor air is important to risk managers because generally EPA does not
clean up to concentrations below natural or anthropogenic background levels (EPA2003e).
These statements hold true for residential and non-residential buildings.
Section 6.3.5 of this Technical Guide provides specific recommendations about howto evaluate
background concentrations. Section 7.4 of this Technical Guide provides clarifications and
recommendations about applying the methods in Section 6.3.5 to informing risk management
decisions and recommendations. The Glossary in this Technical Guide defines various terms
and types of vapor sources to foster a common understanding of EPA's approach and
recommendations.
This section summarizes EPA's general recommendations to considerin making decisions
about evaluating and addressing potential vapor intrusion fornonresidential buildings84 pursuant
to CERCLA and RCRA, including decisions that a response action or corrective action is not
currently warranted.
When evaluating nonresidential buildings at sites that have subsurface contamination with
vapor-forming chemicals, EPA generally recommends that building owners or operators (e.g.,
lessees) be contacted for information about vapor-forming chemicals used or stored or
otherwise present in the building, the types of building occupants potentially exposed to
subsurface vapor intrusion, as well as any training, equipment, or engineering controls to
mitigate inhalation exposures. EPA recommends that information be provided to building
owners concerning the potential for vapor intrusion so that this information can be
communicated to building employees, tenants, and other occupants. Building occupants
include, but are not limited to, facility employees, visitors, customers, suppliers, and building
maintenance personnel.
Generally, EPA recommends the following factors be considered when making decisions
pertaining to vapor intrusion at nonresidential buildings, including as to whether indoor air
sampling, soil gas sampling underneath the building, or interim measures to mitigate vapor
intrusion to reduce associated indoor air exposures for a nonresidential building may be
warranted:
As used in this Technical Guide, the phrase "nonresidential buildings" may include, but is not limited to, institutional
buildings (e.g., schools, libraries, hospitals, community centers and other enclosed structures forgathering, gyms and
other enclosed structures for recreation), commercial buildings (e.g., hotels, office buildings, many (but not all) day
care facilities, and retail establishments); and industrial buildings where vapor-forming substances may or may not be
routinely used or stored.
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1) The types of populations potentially exposed to vapor-forming chemicals in the indoor air
of the nonresidential building, including, for example, whether:
a) Individuals are or may be present under current or reasonably expected future
conditions who would not likely anticipate any chemical exposures (e.g., office
workers, visitors, customers, suppliers, and other members of the general public)
and may not benefit fully from hazard communication programs and otherwork
practices in place to foster protection of workers who use chemicals, if any.
b) Sensitive populations are or may be present undercurrent or reasonably expected
future conditions, who may have increased susceptibility or vulnerability.
2) The potential for vapor intrusion versus background vapor sources (See Glossary) to
contribute to indoor air concentrations of vapor-forming chemicals found in the
subsurface. Questions to consider include, for example:
a) Can subsurface vapor intrusion be identified as a potential cause of unacceptable
human health risk to building occupants (see Section 5 for further discussion about
the preliminary analysis stage and Section 7 for further discussion and definition of
acceptable versus unacceptable human health risk)?85
b) Can subsurface remediation (e.g., excavation of contaminated soil or soil vapor
extraction beneath the subject building) that is planned or underway reduce human
health risk from vapor intrusion within a time frame that is protective for any potential
current or near-term exposures in the building?
c) Is there a known source(s) of one or more vapor-forming chemical(s) - see Section
3.1 -in indoor air in the nonresidential building other than vapor intrusion (e.g.,
indoor use and storage of chemicals, which would constitute a 'background' vapor
source(s) and contribute to indoor air exposure concentrations; see Sections 2.7 and
6.3.5 for further discussion and recommendations about background sources and
concentrations)? If such a vapor-forming chemical(s) is (or are) present:
i. Is(are) it the same as the vapor-forming chemical(s) found in the subsurface?
ii. How does the indoor air exposure concentration(s) arising from the indoor vapor
source(s) compare to the indoor air concentration(s) estimated or reasonably
expected to arise from vapor intrusion?86
Information on'background' contributions of site-related, vapor-forming chemicals in
indoor air is important to risk managers because generally EPA does not clean up to
concentrations below natural or anthropogenic background levels (EPA 2002e).
EPA's recommended approaches to human health risk assessment are provided in Sections 7.4 and 7.5 of this
Technical Guide.
EPA's recommended approaches to distinguishing and considering 'background' are provided in Sections 6.3.5
and 7.4.2 of this Technical Guide.
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3) Any existing or planned engineering or institutional control(s) in the building or any
industrial hygiene/occupational health program that addresses workplace inhalation
exposures and its scope. Questions to consider include, for example:
a) Do work practices and engineering controls currently in place ensure protection87 of
all building occupants who may be exposed via the vapor intrusion pathway?
b) Are enforceable institutional controls (ICs) or other control mechanisms in place to
ensure that current land use and workplace practices will be sustained and will
remain protective regarding indoorair exposures from vapor intrusion to all building
occupants? Have these ICs and control mechanisms been communicated to all
appropriate parties and documented to EPA? Can they be readily monitored and, if
necessary, be enforced?
EPA recommends documenting any decision not to undertake investigation or mitigation for
vapor intrusion in a nonresidential building, as well as any decision to pursue such activities.
EPA may consider reviewing these decisions, as appropriate and consistent with applicable
statutes and regulations and considering EPA guidance,88 if the land use changes or new
information becomes available that suggests circumstances supporting past risk management
decisions have changed and prompt the need to revisit those decisions. It is recommended that
EPA request from property owners and building tenants/operators timely notification of
significant changes in building ownership, uses, access by the general public, or building
construction (e.g., renovations), which may affect exposure of occupants and related risk
management decisions pertaining to potential vapor intrusion assessment and mitigation,
subsurface remediation, or ICs.
Regardless of decisions about indoorair sampling, soil gas sampling underneath the building, or
interim measures to mitigate vapor intrusion, EPA89 may proceed, consistentwith applicable
federal statutes and regulations (see Section 1.2) and considering EPA guidance, with activities
such as the following, where appropriate:
Subsurface investigation to delineate the areal extent of a subsurface vapor plume.
Subsurface remediation to reduce or eliminate subsurface sources of vapors-forming
chemicals to protect human health and the environment.
EPA's recommended approaches to risk management are provided in Sections 7.4 and 7.5 of this Technical Guide.
88 For the Superf und five-year review process, OSWER Drective 9200.2-84 (EPA 2012c) provides a recommended
framework for considering vapor intrusion while evaluating remedy protectiveness.
89 On January 23, 1987, the President of the United States signed Executive Order 12580 entitled, "Superfund
Implementation," which delegates to a number of Federal departments and agencies the authority and responsibility
to implement certain provisions of CERCLA. The policies and procedures for implementing these provisions (e.g.,
carrying out response actions) are spelled out in the NCP The provisions of Executive Order 12580 appear at 52
Federal Register 2923. Atfederal facilities on the NPL, EPA may not be the lead agency, but does have oversight
responsibilities pursuant to CERCLA Section 120.
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5.0 PRELIMINARY ANALYSIS OF VAPOR INTRUSION
When a site is first identified and evaluated for vapor intrusion,90 the amount, utility, and
reliability of available information may be limited. A preliminary analysis utilizes available and
readily ascertainable information to develop an initial understanding of the potential for human
health risk to be posed by vapor intrusion, which would typically be performed as part of an
initial site assessment.
This section describes EPA's recommended information, approaches, and practices for
conducting preliminary analyses for vapor intrusion using pre-existing and readily ascertainable
information to develop an initial understanding of the vapor intrusion potential at a site. This
section:
Explains the recommended types of information that generally can be obtained when a
site is first considered for vapor intrusion (see Sections 5.1, 5.3, 5.4, and 5.5).
Identifies some of the site conditions for which prompt action is generally warranted (see
Section 5.2).
Illustrates some of the site conditions for which further evaluation of the vapor intrusion
pathway might be warranted (see Sections 5.3, 5.4, and 5.5).
Describes the recommended approaches to evaluating the reliability of pre-existing
information, including any sampling data (see Sections 5.1 and 5.5).
Depending upon the nature and reliability of the available information, it may be possible to
determine whether a vapor intrusion investigation (see Section 6) or a response action (see
Sections 7 and 8) is warranted. If the available information is not reliable or adequate for these
purposes, however, additional data collection generally is recommended.
5.1 Assemble, Evaluate, and Review Available Information
The recommended first step in a preliminary analysis generally entails assembling and
reviewing relevant information that is available at the time for the site. At a minimum, EPA
recommends that information about potential subsurface sources of vapors and the presence
and current use(s) of nearby buildings be developed and evaluated. For some sites, such as
sites being evaluated for redevelopment (EPA 2008a), information about contiguous or nearby
facilities also may be relevant, because vapors can encroach from nearby facilities due to
migration of contaminated groundwaterorsoil gas, even though vapor-forming chemicals may
not have been used at the subject site.
The following recommended types of information are often available through documents (e.g.,
federal, state, tribal and local government records) or through interviews with individuals
90 A site may be identified, for example, based on reports to the National Response Center, citizen complaints or
inquiries, state agency referrals, or other information (e.g., site history, land use, site inspections) obtained by EPA.
At a brownfield site subject to an EPA grant, subsurface contamination may be discovered as a result of pre-
acquisition investigation by prospective purchasers, during site redevelopment, or at other project stages.
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knowledgeable about the facility or site and surrounding area (e.g., past and present owners,
operators and occupants; area residents or workers):
History and descriptions of the types of operations and activities that occurred on or near
the site and nearby properties.
Information or records about the types of chemicals that may have been used or
disposed of at the site and nearby properties or are currently used and disposed at the
site.
Information about the site and nearby properties, such as the occurrence of odors,
reports of dumping liquids, observations of unreported waste disposal practices, or other
indications of chemical presence and release.
Adverse physiological effects reported by building occupants (e.g., dizziness, nausea,
vomiting, confusion).
Evidence of subsurface intrusion of groundwater (e.g., wet basements) reported by
building owners or occupants.
Such information usually can be reviewed and weighed together to assess whether vapor-
forming chemicals (see Section 3.1) were or are being used, stored, or handled at or nearthe
site and were or may have been released to the subsurface environment.
In addition, the following types of information may be available through documents, interviews
with individuals knowledgeable about the facility or site, or reconnaissance and site inspection:
Locations, ownership, occupancy, and intended and actual use(s) of buildings on or near
the site.
Current and reasonably anticipated future land use on and near the site.
Location of subsurface utility corridors.
Evaluation of such information usually can help determine whether humans are present
currently or are reasonably expected to be present in the future, and who may become exposed
to any intrusion of vapors from the subsurface into a building(s). Zoning, land use planning, and
related information may also need to be consulted to identify reasonably anticipated future land
use and building types in areas where buildings do not exist or to ascertain whether reasonably
anticipated uses of existing buildings are likely to change (EPA 201 Oc).
EPA recommends evaluating the available data to identify any data gaps for purposes of the
preliminary analysis. For example, has the history of operations and primary activities been
established for the site and all contiguous properties, including currently vacant land? To the
extent that there are significant data gaps, EPA recommends that additional data gathering
(e.g., interviews, records review) generally be planned and conducted.
EPA also recommends evaluating the available data to assess its reliability and internal
consistency. For example, if the available information about operations and activities at a
specific property comes only from area residents, EPA recommends additional efforts to
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identify, contact, and interview current and past owners to obtain and corroborate this
information. Also, if anecdotal information about current activities at a specific property is in
conflict with common knowledge about local zoning, EPA recommends that additional data
gathering and evaluation be identified (e.g., con tact property owner), planned, and conducted to
resolve the inconsistency.
Section 5.5.1 describes additional considerations for evaluating the reliability of sampling data
that may be available for some sites at the preliminary analysis stage.
5.2 Identify and Respond to Conditions that Warrant Prompt Action
The following conditions may indicate a need for prompt action, including follow-up evaluations
to determine whether urgent intervention is warranted to eliminate, avoid, reduce, or otherwise
address a human health hazard:
Odors reported by occupants, particularly if described as "chemical," "solvent," or
"gasoline." The presence of odors does not necessarily correspond to an unacceptable
human health risk due to vapor intrusion, and the odors could be the exclusive result of
indoor vapor sources; however, it is generally prudent to investigate any reports of odors
as the odor threshold for some vapor-forming chemicals exceeds their respective lower
explosive limit (LEL) or health-protective concentrations for short-term or acute
exposure.
Physiological effects reported by occupants (e.g., dizziness, nausea, vomiting,
confusion, etc.). These effects may or may not be due to subsurface vaporintrusion (or
even indoor vapor sources); however, it is generally prudent to investigate any such
reports.
Wet basements in areas where groundwater is known to contain vapor-forming
chemicals (see Section 3.1) and the associated water table is shallow enough that the
basements are prone to groundwater intrusion or flooding. This condition is particularly
important where there is evidence of light NAPL (LNAPL) on the water table directly
below the building or direct evidence of intrusion of liquid-phase contamination (i.e.,
liquid chemical or dissolved in water) inside the building.
EPA generally recommends testing of indoorair (see Sections 6.4.1 and 6.3.4) as soon as
practical in buildings where:
chemical odors or physiologic effects are reported and there is a credible information to
suggest that a release to the subsurface environment may be a contributing factor, or
intruding contaminated groundwater is reported and observed.
Likewise, EPA generally recommends testing of unoccupied structures for explosive gases as
soon as practical where chemical odors are reported and there is a credible information to
suggest that a release to the subsurface environment may be a contributing factor.
Section 7.4 provides EPAs approach and recommendations for identifying when human health
risks are "unacceptable." Section 7.5.2 describes EPAs recommended approaches to
identifying concentration levels indicating a potential need for prompt response action. Section
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8.2.1 identifies potential response actions to reduce or avoid these threats promptly, when the
results of testing reveal threats or potential threats warranting prompt response action.
EPA recommends health and safety planning for all building- or site-specific actions, as
discussed further in Section 6.2, which considers expected work conditions and anticipated
hazards.
5.3 Determine Presence of Structures and Vapor-forming Chemicals
For purposes of this Technical Guide and as reflected in the conceptual model of vapor intrusion
(Section 2), the vapor intrusion pathway is referred to as "complete" for a specific building or
collection of buildings when the following five conditions are met under current conditions:
1) A subsurface source of vapor-forming chemicals is present underneath or nearthe
building(s) (see Sections2.1, 6.2.1, and6.3.1);
2) Vapors form and have a route along which to migrate (be transported) toward the
building(s) (see Sections 2.2 and 6.3.2);
3) The building(s) is (or are) susceptible to soil gas entry, which means openings exist for
the vapors to enter the building(s) and driving 'forces' exist to drawthe vapors from the
subsurface through the openings into the building(s) (see Sections 2.3 and 6.3.3);
4) One or more vapor-forming chemicals comprising the subsurface vaporsource(s) is (or
are) present in the indoor environment (see Sections 6.3.4 and 6.4.1); and
5) The building(s) is (or are) occupied by one or more individuals when the vapor forming
chemical(s) is (or are) present indoors.
EPA recommends that site managers also evaluate whether subsurface vapor sources that
remain have the potential to pose a complete vapor intrusion pathway in the future if site
conditions were to change (e.g., reasonably expected occupancy or construction in the future of
a building above or near a subsurface vapor source). A complete vapor intrusion pathway
indicates that there is an opportunity for human exposure, which warrants further analysis (see
Section 7.4) to determine whether there is a basis for undertaking a response action(s) (see
Section 7.7).
At the preliminary assessment stage, the available information may not be sufficient to evaluate
whether all five conditions are present under current or reasonably expected future conditions.
EPA recommends, however, that readily ascertainable information be reviewed for purposes of
assessing whether the first and fifth conditions are present; that is:
A subsurface source of vapor-forming chemicals is present or is reasonably expected to
be present (e.g., in contaminated groundwater, soil, or sewer lines or from a primary
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vapor release).91 Section 3.1 describes chemicals that have the potential to pose an
unacceptable human health risk through the vapor intrusion pathway. In the absence of
environmental sampling data, the potential presence of vapor-forming chemicals in the
subsurface may be inferred from site information, as identified in Section 5.1 (e.g., site
history).
At least one building is present or is reasonably expected to be constructed in the future
above or "near" the subsurface vaporsource(s), which is or could be occupied by
humans. For purposes of this Technical Guide and its recommendations for evaluating
human health risk posed by vapor-forming chemicals, "building" refers to a structure that
is intended for occupancy and use by humans. This would include, for instance, homes,
offices, stores, commercial and industrial buildings, etc., but would not normally include
sheds, carports, pump houses, or other structures that are not intended for human
occupancy. However, where the assessment identifies the potential for methane or other
potentially explosive vapors to be present in the subsurface, EPA recommends
reviewing readily ascertainable information for purposes of assessing whether non-
occupied structures (including, but not limited to, sewers, pits, and subsurface drains)
are present, which may also accumulate vapors, in addition to occupied and non-
occupied buildings. Existing buildings (and non-occupied structures) can be identified
during inspections of the land areas overlying and near subsurface vaporsources. The
potential presence of buildings in the future may be inferred from site information, such
as identified in Section 5.1. See Section 6.2.1 forfurtherdiscussionon which buildings
and non-occupied structures are considered "near" for purposes of a preliminary
analysis.
If the available information is deemed reliable, well documented, and sufficient (see Section 5.1)
and indicates that neither of these conditions is met, then further vapor intrusion assessments
are not generally warranted.92
Example: From 1920 to 1931, the ABC Mining Company obtained and shipped iron ore
from a local deposit. Ore from the mine was shipped by rail to a different location where
it was milled and processed to extract the metal. Although no company records are
available for the mine, a review of mining techniques indicates that solvents and other
vapor-forming chemicals were not used in the mining process during the 1920s and
1930s. Former mining structures have been removed, and the site is currently vacant.
The city has proposed redeveloping the site with bike and hiking trails but no buildings or
other structures for storage or site maintenance support. Based on the information and
Q1
As noted in Section 2.1, the primary contamination source need not be on the property of interest to pose a vapor
intrusion problem. The primary source(s) of vapor-forming chemicals (e.g., contaminated soil, leaking tanks) may be
present on a neighboring property or on a property some distance away. Even "greenspace" properties that have not
previously been occupied or developed may contain subsurface contamination by vapor-forming chemicals due to
migrating plumes of contaminated groundwater or migrating soil gases. Therefore, EPA recommends that the
potential for vapor intrusion be considered at all properties being considered for redevelopment (EPA 2008a) or
proximate to industrial and commercial use areas.
92 Consistent with federal environmental protection statutes, regulations, and OSWER guidance, a subsurface
investigation may still be warranted for non-volatile substances or for other potential exposure pathways such as
those identified in Section 1.3.
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findings, the need for further assessment of the vapor intrusion pathway due to mining-
related contamination is not indicated.
If, on the other hand, there is reliable evidence to indicate that a release of vapor-forming
chemicals to the subsurface has occurred (e.g., environmental sampling data indicate
detectable levels of a vapor-forming chemical(s) in potential source media)93 or may have
occurred underneath or near a property, then EPA recommends further vapor intrusion
assessment in areas where buildings are present or future buildings could be constructed,
including development of a conceptual site model (see Section 5.4) and investigation of site-
specific conditions (see Section 6).
Example: The XYZ Recycling Center site was used from 1963 to 1984 for the collection
and recycling of industrial solvents and other fluids. The site was repeatedly cited by the
state and city for improper handling and disposal of solvents, and was closed in 1985.
Groundwaterdata indicate the presence of multiple chlorinated hydrocarbons. Buildings
overlying the contaminated groundwater are currently used mainly for storage of non-
chemical goods, but the site has been proposed for future residential or commercial
redevelopment. Based on the foregoing information and findings, further assessment of
the potential for vapor intrusion is warranted, possibly including risk-based screening of
the groundwater data (see Section 6.5).
If a release of vapor-forming chemicals to the subsurface is known or suspected to have
occurred at or near the site, but buildings are not present and none are reasonably anticipated
in the future (e.g., the contaminated source underlies an open space, recreational area, or
wildlife refuge), then further vapor intrusion assessments may not be appropriate under current
conditions. It may be appropriate, however, to establish an institutional control (1C) requiring a
vapor intrusion investigation or building mitigation94 in the future, in case land use changes. ICs
for building mitigation and subsurface vapor source remediation are discussed further in
Sections 3.3 and 8.6 of this Technical Guide.
Existing guidance and practice pursuant to CERCLAand RCRA corrective action (CA)
recognize and entail various phases of subsurface or site characterization, including a site
investigation to determine the full nature and extent of contamination at a site, quantify risks
posed to human health and the environment, and gather information to support the selection
and implementation of appropriate remedies. On this basis, a subsurface investigation may be
warranted at some point to characterize subsurface contamination and assess the need for
subsurface remediation to protect the environment and human health for potential exposure
pathways other than vapor intrusion (such as those identified in Section 1.3). For example, site
investigations to characterize the nature and extent of groundwater contamination and support
assessments of risk to human health through the ingestion pathway are typically conducted,
consistent with federal statutes and regulations (e.g., CERCLAand RCRA) and considering
EPA guidance.
93 Section 6.5 provides information on how such data may be used in a quantitative fashion to screen the site further.
94 If, for example, a developer is considering acquiring and building on land that contains subsurface contamination
with vapor-forming chemicals, the developer could retrofit existing buildings or build new buildings with vapor
mitigation systems withoutfirst conducting an extensive vapor intrusion investigation (see Sections 3.3 and 7.8).
Section 8.2.3 identifies additional approaches and considerations for new buildings.
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5.4 Develop Initial Conceptual Site Model
EPA recommends that the planning and data reviewteam develop an initial conceptual site
model (CSM) for vapor intrusion when the preliminary analysis indicates the presence of
subsurface contamination with vapor-forming chemicals underlying or near buildings. The initial
CSM (and any subsequent refined CSM) can be used to support evaluations of the adequacy of
the available site-specific information, to guide any vapor intrusion investigations (see Sections
6.2 and 6.3), and to support data selection for risk-based screening (see Section 6.5). The CSM
can also provide useful information for supporting prompt development of a strategy for
early/interim response actions (see Sections 7.8 and 8.2).
The remainder of this section discusses recommended information that can be useful for
developing a CSM. Note that some of the recommended information may not be readily
available when a site is first considered for vapor intrusion. Although the CSM may be updated
iteratively (and interim mitigation measures may be undertaken) as the vapor intrusion
investigation unfolds, EPA recommends completing the CSM before making final risk
management decisions for a given site (see Section 7).
As discussed in Section 5.3, the available information may not be sufficient at the preliminary
analysis stage to evaluate whether the vapor intrusion pathway is complete under current or
future conditions. Therefore, the initial CSM for vapor intrusion is likely to be incomplete. EPA
recommends, however, that the initial CSM for vapor intrusion portray the current understand ing
of site-specific conditions pertaining to the vapor intrusion pathway. Ideally, at a minimum, the
initial CSM will address:
Nature (i.e., type, chemical composition), location, and spatial extent of the source(s)of
vapor-forming chemicals in the subsurface (see Sections2.1 and 6.3.1, for example).
For example, it is useful to knowwhich vapor-forming chemical(s) primarily comprise the
subsurface vapor source95 and whether it is also capable of posing explosion hazards. It
is also useful to know whether vapor-forming chemicals are present in groundwater,
vadose zone soils, sewer lines, and/or some other source underneath or near buildings.
Location, use, occupancy, and basic construction (e.g., foundation type) of existing
buildings.
The CSM can be updated as additional information is obtained through investigation (Section 6)
and building surveys (Section 6.4.1).
EPA recommends the CSM also portray the current understanding of the hydrologic and
geologic setting in and around the subsurface vapor source(s) and the buildings, which is
expected to influence vapor migration and attenuation in the vadose zone (see Sections 2.2 and
6.3.2, for example). When these conditions are not well established from existing information,
and the preliminary analysis indicates the presence of subsurface contamination with vapor-
EPA also recommends that the CSM identify any site-specific chemicals of concern that may be biodegradable and
identify and summarize information and data pertaining to the possible role of biodegradation in situ'm limiting vapor
migration in the vadose zone (see Section 6.3.2) or generating hazardous, volatile degradation products (e.g.,
methane from anaerobic biodegradation, vinyl chloride as a byproduct of PCE or TCE biodegradation).
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forming chemicals underlying or near buildings, EPA recommends that a detailed vapor
intrusion investigation be scoped and conducted to address these data gaps (see Section 6.3).
Furthermore, EPA recommends the CSM identify known or suspected preferential migration
routes that could facilitate vapor migration to greater distances and at higher concentrations
than otherwise expected. EPA recommends that buildings with significant preferential migration
routes be evaluated closely. For the purposes of this Technical Guide, a preferential migration
route is a naturally occurring subsurface feature or anthropogenic (human-made) subsurface
conduit that is expected to exhibit little resistance to vapor flow in the vadose zone (i.e., exhibits
a relatively high gas permeability) orgroundwaterflow(i.e., exhibits a relatively high hydraulic
conductivity), depending upon its location and orientation relative to the water table and ground
surface, thereby facilitating the migration of vapor-forming chemicals in the subsurface and/or
into buildings.96 Naturally occurring examples include fractures and macropores, which may
facilitate a preferential route for either the vertical or horizontal migration of source materials
and/or vapors. Anthropogenic examples include sewer lines and manholes,97 utility vaults and
corridors, elevator shafts, subsurface drains, permeable fill, and underground mine workings
that intersect subsurface vapor sources or vapor migration routes. In highly developed
residential areas, extensive networks of subsurface utility corridors may be present, which can
significantly influence the migration of contaminants. A preferential migration route can be a
"significant" influence on vapor intrusion when it is of sufficient volume and proximity to a
building that it may be reasonably anticipated to influence vapor migration towards or vapor
intrusion into the building. Significant vertical routes of preferential migration may result in higher
than anticipated concentrations in the overlying near-surface soils, whereas significant
horizontal routes of preferential migration may result in elevated concentrations in areas on the
periphery of subsurface contamination (see Section 6.2.1).
CSMs for vapor intrusion assessments often need to considertwo distinct exposure situations:
1) At some sites and contaminated locations, there are concerns as to whether vapor
intrusion may pose a human health risk to current occupants of an existing building(s).
For this situation, EPA recommends that building-specific information be available to
support the CSM, which may be obtained through a building survey (see Section 6.4.1,
for example).
2) At other sites and contaminated locations, buildings are not present, but are expected to
be constructed, and building-specific information may not be available to support the
CSM. For this situation, the CSM may need to consider a hypothetical building
constructed anywhere over (or near) the subsurface vapor source.
96 For purposes of this Technical Guide, preferential migration routes are distinguished from adventitious and
intentional openings in a building that may also facilitate vapor entry from the subsurface (see Section 2.3), but which
are expected to typically be present in all buildings (e.g., cracks, seams, interstices, and gaps in basement floors and
walls or foundations; perforations due to utility conduits).
97 In addition to receiving direct discharges of aqueous and chemical wastes from commercial and industrial
operations, sewers can be indirect receptacles of subsurface contamination via infiltration of NAPL, soil gas, or
contaminated groundwater through cracks in piping and manholes (see Section 2 of this Technical Guide, for
example, for further discussion).
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In general, CSMs identify the potentially exposed populations, potential exposure routes, and
potential adverse health effects (i.e., toxicity) arising from these exposures. As such, EPA
recommends the CSM also identify and consider sensitive populations, including but not limited
to:
Elderly,
Women of child-bearing age,
Infants and children,
People suffering from chronic illness, or
Disadvantaged populations (i.e., an environmental justice situation).
By definition and as noted in Section 2, the exposure route of general interest for vapor intrusion
is inhalation of vapors in indoor air and the human population of primary interest is comprised of
individuals living or working in or otherwise occupying a building subject to vapor intrusion.
However, EPA also recommends that the CSM identify any site-specific chemicals of concern
that have potential for explosion hazards (e.g., methane) or for posing other routes of exposure
(e.g., dermal exposure to shallow contaminated groundwaterseeping into a basement).
EPA recommends that the CSM also identify and characterize suspected sources of site-
related, vapor-forming chemicals that are also found in ambient air in the site vicinity. In some
situations, site-related contamination has the potential to impact ambient air with the same
vapor-forming chemicals that pose a threat from vapor intrusion. Forexample, contamination of
shallow soil or groundwater may release site-related vapor-forming chemicals to ambient air.
EPA recommends the CSM identify any such conditions, which have implications for the scope
and objectives of the overall site investigation, as well as for data evaluation and the human
health risk assessment.
To document current site conditions, EPA recommends that a CSM be supported by maps,
cross sections, and site diagrams, to the extent practical, and that the narrative description
clearly distinguish what aspects are known or determined and what assumptions have been
made in its development.
EPA generally recommends that developing a CSM be incorporated into the first step in EPAs
data quality objective (DQO) process (EPA 2006a). It is rare for a site to have readily available
sources of sufficient information to develop a complete CSM when the vapor intrusion potential
is first considered. Forexample, a detailed site-specific investigation may be necessary to
characterize the full extent of subsurface vaporsources and geologic conditions underlying
nearby buildings (see Sections 6.3.1 and 6.3.2) and to demonstrate the absence of preferential
routes for vapor migration and intrusion. The CSM normally warrants updating as new
information is developed and newquestions are framed and answered. A well-defined, detailed
CSM may also facilitate the identification of additional data needs and development of
appropriate detection limits for laboratory and field analyses, which can support planning of the
detailed vapor intrusion investigation (see Section 6.2) and site-specific human health risk
assessment, if any (see Section 7.4). Sections 6.3, 6.4, 7.1, and 7.2 provide additional
information about data collection and evaluation for purposes of supporting the CSM.
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5.5 Evaluating Pre-Existing and Readily Ascertainable Sampling Data
Sites and adjacent facilities that have been the subject of previous environmental investigations
or regulatory actions may already have data on contaminant concentrations in site media (i.e.,
sampling data) when the vapor intrusion pathway is first considered and evaluated. Some of
these sites and facilities may be undergoing remediation but warrant a vapor intrusion
assessment as a result, for example, of changing toxicity information for vapor-forming
chemicals, as part of a periodic review of remediation effectiveness and protectiveness (if any),
or for other reasons.
If the pre-existing environmental data are deemed reliable and other conditions are met (as
described in the remainder of this subsection and in Section 6.5.2), the sampling data may be
compared to recommended generic vapor intrusion screening criteria (see Section 6.5) for
purposes of developing some preliminary insights aboutthe potential level of exposure and risk
posed by vapor intrusion. Such a screening can, for example, help focus a subsequent vapor
intrusion investigation (see Section 6) or provide support for considering building mitigation as
an early action (see Section 7.8.2), depending upon building- and site-specific circumstances.
Note that some of the site-specific information generally recommended for supporting a risk-
based screening (see Section 6.5.2) may not be available when a site is first considered for
vapor intrusion.
5.5.1 Evaluate Sampling Data Reliability and Quality
To the extent that environmental sampling data are identified for the site or nearby properties,
EPA recommends that these data be evaluated to determine whether they are of sufficient
quality and reliability to support a comparison to recommended generic vapor intrusion
screening criteria (see Section 6.5). Some questions that could be considered when reviewing
historical sampling data include:
How were the samples collected and analyzed? EPA recommends using pre-existing
data when they have been collected and analyzed by methods considered reliable by
today's standards.
How old are the data? Were analyses conducted for all vapor-forming chemicals known
or suspected to be present and reasonably expected degradation products? EPA
recommends using pre-existing data when they can be considered representative of
current conditions.
Were the reporting limits sufficiently lowfor comparison with vapor intrusion screening
criteria? EPA recommends use of pre-existing data with non-detect results only when
they can be considered reliable on this basis.
Were multiple locations sampled to assess spatial variability of the results? Were
multiple sampling events conducted to assess temporal variability of the results? EPA
recommends characterizing spatial and temporal variability to increase confidence in
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data evaluation and decision-making and ensure consideration of a reasonable
maximum vapor intrusion condition.98
EPA also recommends that the reliability of any historical sampling data be assessed by
considering the principles for collecting subsurface and indoor air samples that are described in
Sections 6.3.1 and 6.4 of this Technical Guide. In addition, the EPA's Guidance for Data
Usability in Risk Assessment, Part A (EPA 1992a) outlines a recommended approach for
evaluating whether the data are useable forthe human health risk assessment. As such, its
recommended approach is also worthwhile and complementary for evaluating the quality and
usefulness of historical data collected at a site.
5.5.2 Evaluate Applicability of the VISLs and Adequacy of the Initial CSM
Before performing any comparison of existing sampling data to recommended generic vapor
intrusion screening levels (VISLs) (see Section 6.5), it is important to verify that site-specific
conditions reflect the conditions and assumptions of the generic model underlying the VISLs,
which are summarized in Section 6.5.2. To verify that the generic vapor intrusion model applies,
there is a need for basic knowledge of the subsurface source of vapors (e.g., location, form, and
extent of site-specific vapor-forming chemicals) and subsurface conditions (e.g., soil type in the
vadosezone, depth to groundwaterforgroundwater sources), which are important elements of
the CSM (see Section 5.4). When these subsurface data are not available, EPA recommends
they be collected (i.e., initiate a vapor intrusion investigation; see Section 6.3.2, for example)
before relying upon risk-based screening using pre-existing sampling data.
5.5.3 Preliminary Risk-based Screening
If reliable pre-existing sampling data are available and an adequate CSM has been developed
(i.e., sufficient subsurface characterization information exists to adequately characterize the
locations, forms, and extent of site-specific vapor-forming chemicals and general subsurface
conditions (e.g., hydrologic and geologic setting in and around the source(s) and the buildings)),
then a risk-based screening may be useful to obtain some preliminary insights about the
potential level of exposure and risk posed by vapor intrusion.
Example: A prospective developer of a vacant lot with no history of onsite chemical use is
interested in evaluating the potential for vapor intrusion in the future due to potential
migration onto the lot of a plume of contaminated groundwateremanating from another
property. The extent and nature of this off-property plume have been adequately and
recently characterized and geologic conditions near (but not on) the lot have been
characterized, as documented in a publicly available report(s). In this circumstance, it may
be possible to support a preliminary screening and obtain some useful insights. For
example, if the maximum concentration of each chemical of concern in the off-property
plume of contaminated groundwatercurrently and in the future is less than the generic
chemical-specific screening level for groundwater, then vapor intrusion may not be a future
EPA recommends basing the decision about whetherto undertake response action for vapor intrusion (i.e., a
component of risk management; see Section 7.4) on a consideration of a reasonable maximum exposure (e.g., EPA
1989, 1991 a), which is intended to be a semi-quantitative phrase, referring to the lower portion of the high end of the
exposure distribution (see Glossary).
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concern on the vacant lot, provided there are sufficient data to document that conditions on
the vacant lot are consistent with the generic model behind the vapor intrusion screening
levels, as described in Section 6.5.2.
Depending upon lot-specific circumstances, additional data collection or evaluation, possibly
including on-lotsite characterization, maybe warranted (i.e., proceed to a detailed vapor
intrusion investigation) to verify that the expected conditions hold true (e.g., hydrogeologic
conditions on the vacant lot are consistent with the generic model behind the vapor intrusion
screening levels). EPA generally also recommends consideration of the vapor intrusion
pathway during the development planning or initial post-construction stage (e.g., pre-
emptive mitigation - Sections 3.3 and 7.8; mathematical modeling, where parameters are
chosen to represent conditions that give a high-impact case - Section 6.6; indoor air testing
- Section 6.4.1 to confirm the screening results based upon the groundwatersource data)
before making final risk management decisions.
This example reinforces the following general recommended guidelines:
EPA generally recommends that site-specific data be collected and evaluated to verify
that the subject property reflects the conditions and assumptions of the generic model
underlying the VISLs (see Section 6.5.2).
EPA generally recommends that multiple lines of evidence (e.g., hydrogeologic
information in addition to sampling data) be collected and weighed together in supporting
assessments of the vapor intrusion pathway (see Sections 7.1,7.2, and 7.3 for further
information).
Multiple rounds of groundwater (orsoil gas) sampling results can be useful in supporting
conclusions that a specific vapor source is stable or shrinking and/or is not expected to
pose a vapor intrusion concern (see Sections 6.3.1 and 6.4.5) under reasonably
expected future, as well as current, conditions.
Similar recommended guidelines may be appropriate in situations where vapor intrusion
potential is being evaluated as part of a periodic review of an existing remedy (prompted, for
example, by recent construction of a new building over a contaminated plume that is undergoing
remediation) (EPA2002b, 2012c).
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6.0 DETAILED INVESTIGATION OF VAPOR INTRUSION
EPA recommends that the planning and data reviewteam plan and conduct a site investigation
for vapor intrusion when the preliminary analysis (Section 5.3) indicates the presence of
subsurface contamination with vapor-forming chemicals underlying or near buildings.
This section describes EPA's generally recommended approaches and practices for conducting
detailed vapor intrusion investigations, which typically entail collecting and weighing multiple
lines of evidence to characterize the vapor intrusion pathway. Specifically, this section:
Identifies that a wide variety of scenarios may be encountered among sites investigated
for potential vapor intrusion, which necessitates site-specific approaches to scoping
investigations and sequencing investigation phases and objectives (Section 6.1);
Provides EPA's recommendations for planning, scoping, and conducting vapor intrusion
investigations (Sections6.2, 6.3, and 6.4);
Presents EPA's recommended screening levels for vapor intrusion and describes EPA's
recommended uses of risk-based screening and suggested interpretation of the results
(Section 6.5); and
Provides recommendations fordeveloping and using mathematical models in vapor
intrusion assessments (Section 6.6).
Section 7 describes EPA's generally recommended approaches and practices for determining,
on the basis of the investigation results, whether the vapor intrusion pathway poses a potential
human health risk to building occupants under current and reasonably expected future
conditions and whether response actions are warranted for vapor intrusion mitigation at
individual facilities, buildings, or sites.
6.1 Common Vapor Intrusion Scenarios
Vapor intrusion scenarios can be quite varied, owing to the possible combinations of:
Multiple hazardous chemicals that can form vapors.
Multiple forms in which these chemicals may be released to or present as contaminants
in the subsurface, for example:
o Residual NAPL and adsorbed-phase chemicals, including LNAPLs that are less
dense than water and DNAPLs that are denser than water.
o Dissolved-phase chemicals in groundwateror soil moisture.
o Primary vapor releases (e.g., from chemical vapor transmission lines).
The variety of geologic and hydrologic characteristics and conditions in the subsurface
environment in which this contamination may occur.
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The variety of buildings (in terms of size, age, condition, and use) and current or
expected land use settings (e.g., residential, commercial, industrial) that may be subject
to vapor intrusion from such subsurface contamination.
Circumstances under which subsurface contamination is found or suspected and
investigated (e.g., brownfield redevelopment, citizen reports/complaints, reported
release)
The variety of sources that may contribute to vapor concentrations in ambient air and
may serve as indoor vapor sources unrelated to vapor intrusion.
A few of the possible scenarios are illustrated in Figure 2-1. Many more can be inferred from the
conceptual model of vapor intrusion discussed in Section 2. Some of the common scenarios
where vapor intrusion has been documented to occur include:
Groundwater contaminant plumes in shallowaquifers underlying residential and
nonresidential buildings. Many well-known vapor intrusion sites are in this category, in
part because there is generally a greater opportunity to have multiple buildings overlying
the vapor source. Specific sites and buildings normally can be prioritized and
distinguished based upon their potential for vapor intrusion, which generally would
depend upon a number of site-specific factors, such as:
o strength, proximity, and extent of the vapor source emanating from shallow
groundwater (see Sections 2.1 and 5.4);
o the potential for significant attenuation of vapor migration due to geologic,
hydrologic, or biochemical conditions in the vadose zone (see Sections 2.2 and
5.4);
o the potential for significant attenuation of the contaminant plume due to geologic,
hydrologic, or biochemical conditions in the saturated zone; and
o type(s), characteristics and structural condition of the overlying building(s) (see
Sections 2.3, 2.4, and 5.4).
Soil contamination in the vadose zone underlying commercial or industrial buildings.
Typically, one or a few buildings may be threatened by potential vapor intrusion. Specific
buildings and sites normally can be prioritized and distinguished based upon their
potential for vapor intrusion, which generally would depend upon a number of site-
specific factors, such as:
o strength, proximity, and extent of the vadose zone source (see Sections2.1 and
5.4);
o the potential for attenuation of vapor migration due to geologic, hydrologic, or
biochemical conditions in the vadose zone (see Sections 2.2 and 5.4); and
o type(s), characteristics and structural condition of the overlying building(s) (see
Sections 2.3, 2.4, and 5.4).
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Sites with residual wastes (e.g., landfills, former manufactured gas plants, former oil
production fields) underlying or near buildings. The potential for methane formation may
more frequently warrant additional consideration for sites with residual wastes than for
contaminated groundwater plumes.
EPA's recommended approaches and practices for vapor intrusion investigations aim to be
flexible and adaptable to a wide range of reasonably expected scenarios and are not intended
to be prescriptive or exhaustive for any specific scenario.
6.2 Planning and Scoping
Before information or data are collected, EPA generally recommends conducting systematic and
thorough planning during which performance or acceptance criteria are developed for the
collection, evaluation, or use of these data (EPA 2006a)." EPA recommends the data quality
objective (DQO) process as the appropriate systematic planning process for its decision-making
and has issued guidance for its application to hazardous waste site investigations pursuant to
CERCLAand RCRA(EPA2000a). 100When appropriately conducted, planning provides greater
assurance that the data collected will fulfill specific project needs and that mitigation and
subsurface remediation options will be considered early in the process.101 A clear and logical
plan will often facilitate communication with building owners, occupants, and other stakeholders.
Given these considerations, a thorough planning process, guided by a CSM, is usually
advisable for detailed vapor intrusion investigations. Figure 6-1 provides a diagram to illustrate
such planning and scoping. The initial stages of planning would typically entail gathering readily
available existing information and formulating an initial CSM, as described in Section 5.4. The
CSM portrays the current understanding of site-specificconditions, including the nature and
extent of contamination, contaminant fate and transport routes, potential "receptors" and
contaminant exposure pathways. The term "conceptual" merely reflects that the model need not
be entirely quantitative and mathematical; it does not, however, denote a simplistic or
incomplete understanding of site conditions. The CSM normally warrants updating as new
information is developed and new investigatory questions are framed and answered.
Subsequent to formulating an initial CSM based on readily available information, the scope for
an initial phase of vapor intrusion investigation would be developed, preferably along with a
logical plan for future directions in response to the reasonably expected outcomes of the initial
investigatory phases and in coordination with the objectives and phasing of the broader site
In situations where imminent threats (see Section 5.2) are known or reasonably expected, the initial planning
process may be more truncated and focused, but careful and thoughtful planning is still recommended.
100
Appendix B provides additional information about EPA's quality system and DQO process.
101 Science and Decisions: Advancing Risk Assessment was prepared by the National Academy of Sciences (MAS)
Committee on Improving Rsk Analysis Approaches Used by the U.S. EPA (NRC 2009) and is commonly referred to
as the "Silver Book." Among other recommendations, the MAS Committee encouraged EPA to focus greater attention
on design in the formative stages of risk assessment, specifically on planning and scoping and problem formulation,
and to view risk assessments as a method for evaluating the relative merits of various options for managing risk,
rather than as an end in itself. Consistent with these recommendations, plausible mitigation and subsurface
remediation options (see Section 8) may warrant consideration during development of vapor intrusion investigation
plans.
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Consider
Scenario and
Conceptual
Site Model -
See Sections 5.4
and 6.1
Consider &
Prioritize
Investigation
Objectives -
See Section 6.3
Identify
"Higher
Priority"
Buildings -sei
Establish
Data Quality
Objectives -
See Appendix C
Scope and Prepare Investigation Workplan (See Section 6.2)
Collect Samples and Complementary Lines of Evidence (See
Sections 6.3, 6.4, and 7.1) - sequence need not follow this order of steps:
Characterize Nature and Extent of Vapor Sources (See Sections 6.3.1,
6.4.4, and 6.4.5)
Test Indoor Air (Sections 6.3.4, 6.3.5, and 6.4.1)
Characterize Vapor Migration in the Vadose Zone (from source to subslab)
(See Sections 6.3.2, 6.4.3, and 6.4.4)
Evaluate Contribution from Background Sources (See Sections 6.3.5 ,
6.4.1, and 6.4.2)
Update/Refine Conceptual Site Model
Identify Data Gaps (Section 5.4) and resolve
inconsistencies, if any, between new site-
specific info and existing CSM (Section 7.2)
Verify boundaries of inclusion zone (Section
6.2.2)
Is site-specific
information
sufficient to support
decision-making?
Data Evaluation
1. Compare Sample Concentrations to Health-based Screening Levels (Section
6.5.4)
2. Weigh Site-specific Lines of Evidence and Assess Their Concordance
(Sections 7.1 and 7.2)
3. Evaluate Whether the Vapor Intrusion Pathway is Complete or Incomplete
(Section 7.3)
4. Conduct and Interpret Health Risk Assessment (Section 7.4)
Exclamation point {!) indicates important milestone for communication and engagement efforts with affected building occupants
and owners.
Figure 6-1 Overview of Planning, Scoping, and Conducting Vapor Intrusion Investigations
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characterization. Initial plans may warrant periodic updates and refinements, particularly when
data outcomes are unexpected and prompt the need to reevaluate the CSM. In each case, EPA
recommends that the investigation work plan include the identification of and basis for the
indoor air screening levels (such as the vapor intrusion screening levels (VISLs)) and/or indoor
air action levels (i.e., level of each vapor-forming chemical of potential concern that would
trigger a response action if exceeded), which would dictate the DQOs for the sampling and
analysis methods. In general, EPA recommends the plan also include a rationale or logic for
where and how the data will be collected and over what duration(s),howthe data will be
interpreted (e.g., weighed with other lines of evidence, compared to risk-based benchmarks),
whether confirmatory sampling will be needed if all sample concentrations are less than the
action levels, whether response action(s) would be triggered if sample concentrations exceed
the target levels, and similar considerations. EPA recommends considering potential health
effects and relevant exposure periods (e.g., chronic versus short-term effects and exposure
durations and scenarios; see Section 7.4) for site-related, vapor-forming chemicals when
developing DQOs and sampling plans for indoor air (see Section 6.4.1, for example)..Sections
6.3 through 6.6 below provide additional information for planning and scoping site-specific
investigations for vapor intrusion assessment.
EPAs fundamental approach to evaluating contaminated sites calls for proceeding in a stepwise
fashion with early data collection efforts usually limited to developing a basic understanding of
the site, as reflected in the CSM.102 Subsequent data collection efforts focus on filling gaps in
the understanding of the CSM and gathering information necessary to evaluate the relative
merits of various options for managing risk. Therefore, EPA generally recommends developing
and implementing an overall vapor intrusion investigation plan in multiple stages or phases.
Such a phased approach encourages the identification of key data needs early in the process to
better ensure that data collection provides information relevant to decision-making (e.g., interim
action to mitigate vapor intrusion and selection of a cleanup plan for subsurface contamination).
In this way, the overall site characterization effort can be scoped to prioritize data collection and
minimize the collection of unnecessary data and maximize data quality.
EPA recommends that the objectives and methods of the investigation be documented,
preferably in a vapor intrusion work plan. An individual work plan may address a single phase or
stage or may address the overall investigation. The vapor intrusion work plan(s) may be
10?
Investigations under CERCLA and RCRA corrective action (CA) explicitly recognize phasing. In these cleanup
programs, the first investigatory phase is an initial site assessment. The purpose of this activity is to gather
information on site conditions (current and historical), releases, potential releases, and exposure pathways.
Investigators use this information to determine whethera response action (e.g., removal action or interim cleanup
measure) may be needed or to identify areas of concern for further study. Information collected during this phase
usually forms the basis for determining whetherthe next stage, site investigation, is warranted. In the RCRA CA
program, the initial site assessment is called the RCRA facility assessment. Under CERCLA, this phase is called the
preliminary assessment/site inspection. The purpose of the second phase, site investigation, is to determine the
nature and extent of contamination at a site, quantify risks posed to human health and the environment, and gather
information to support the selection and implementation of appropriate remedies. In the RCRA CA program, this
phase is known as the RCRA facility investigation. Under the CERCLA remedial program, this phase is referred to as
the remedial investigation. In addition, the site investigation may itself be conducted in multiple stages (or phases).
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incorporated as part of a comprehensive site investigation work plan or as a stand-alone
document, depending upon site-specific circumstances.103
At a minimum, EPA recommends that the components of the work plan(s) include or reference:
Narrative description of the rationale, objective(s), and scope of the investigation.
Summary of the CSM, based upon the current understanding of site conditions.
Scaled map(s) illustrating known extent of subsurface contamination and readily
identifiable landmarks (e.g., streets and buildings).
Media to be sampled.
Number, type, and location of and rationale for proposed sampling locations.
Sampling methods and procedures for each medium.
Analytic method(s) to be used to obtain chemical concentrations and a statement about
whether a stationary or mobile laboratory will be used.
Standard operating procedures of the laboratory and forfield instruments.
Quality assurance project plan (QAPP).
Health and safety plan.104
EPA recommends that planning for vapor intrusion investigations also consider site and building
access agreements, equipment security, and locations of underground utilities.
EPA recommends that the planning, data collection, and data reviewteam(s) for vapor intrusion
investigations generally include:
Individuals with expertise in characterizing subsurface environmental conditions and
interpreting and communicating environmental data.
On-site (field) personnel with appropriate training and experience in hazard identification,
workplace practices to foster health and safety, and recommended sampling protocols.
103
EPA recommends that monitoring programs (see Section 8.4) that assess the performance and effectiveness of
remediation and mitigation systems (see Sections 8.1 and 8.2, respectively) also be documented, preferably in work
plans similar to those recommended herein for characterizing and assessing the vapor intrusion pathway.
All governmental agencies and private employers are directly responsible forthe health and safety of their
employees. This general rule applies to many parties involved in the assessment and cleanup of Superfund sites,
RCRA corrective action sites, and brow nfield redevelopment sites. Standards established pursuant to the
Occupational Safety and Health Act are found in Title 29 of the Code of Federal Regulations (29 CFR), which include
standards for training, hazard communication, and site-specific health and safety plans.
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Individuals with expertise in human health risk assessment to characterize risks posed
by the vapor intrusion pathway.
Individuals with expertise in community involvement and outreach.
Depending upon the complexity of the CSM (see Section 5.4) and site-specific data evaluations,
decision-makers may also find valuable input from individuals with expertise in hydrogeology,
inferential statistics, laboratory analysis methods, and building construction, ventilation, and
operations and individuals knowledgeable about land use planning, zoning, and land
development.
EPA recommends that the scope of investigations within buildings and on individual properties
be contemplated, planned, and implemented with the goal of limiting, to the extent practical,
return visits, which can cause disruption and inconvenience for building occupants and owners.
For example, it may be preferable to collect a comprehensive set of data (e.g.,indoorair, sub-
slab soil gas, and ambient air samples; pressure readings; see Section 6.4) and confirm
information about building occupancy, building usage, heating, cooling, and ventilation (see
Section 6.4.1) in a single mobilization, rather than over separate visits, when the investigation
objectives include indoor air sampling (see Section 6.3.4) or evaluating contributions of
'background' sources on levels of vapor-forming chemicals in indoorair (see Section 6.3.5).
6.2.1 Vapor Intrusion Inclusion Zones
Soil gas concentrations generally decrease with increasing distance from a subsurface vapor
source, and eventually at some distance the concentrations become negligible. The distance at
which soil gas concentrations become negligible is a function of the strength and dimensions of
the vapor source, the type of vaporsource, the soil types and layering in the vadose zone, the
presence of physical barriers (e.g., asphalt covers or ice) at the ground surface, and the
presence of preferential migration routes, among other factors (see, for example, EPA 2012b).
The extent of the site-specific "inclusion zone" for vapor intrusion may also expand in the future,
depending upon:
The age of the chemical release and whether sufficient time has elapsed to allow soil
gas to migrate from the source to its maximum potential extent.105
Whether the subsurface vaporsource is expanding (i.e., is migrating) or rising in
concentration, including hazardous byproducts of any biodegradation.
Because these factors vary among sites, the distance beyond which structures will not be
affected by vapor intrusion should be a site-specific determination.
Recommended Distance for Initial Evaluation. There are limited published empirical data
relating observed indoor air concentrations of subsurface contaminants to distance from a well-
105 EPA (2012b, Section 6.1) presents some information about transient vapor migration after asubsurfacevapor
source is released. Sites with shallow vapor sources(e.g., less than one meter deep) may take only a few hours to a
few days for soil gas to migrate to its maximum potential extent. Sites with deeper vapor sources (e.g., greater than
10 meters deep) may take months or years for soil gas to migrate to its maximum potential extent.
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defined source boundary. However, a buffer zone of approximately 100 feet (laterally or
vertically from the "boundary" of subsurface vapor concentrations of potential concern) generally
has been used in determining which buildings to include in vapor intrusion investigations (i.e.,
which buildings are 'near' a subsurface vaporsourcefor purposes of a preliminary analysis)
when significant surface covers are not present, underthe assumption that preferential vapor
migration routes are absent.106 Specifically, a buffer zone of 100 feet (or approximately two
houses wide) has been suggested by several states for initial evaluation and is supported, in
general, by:
theoretical analyses that assume the absence of a preferential vapor migration route(s)
and that diffusion is the predominant mechanism of vapor migration in the vadose zone
(Lowell and Eklund 2004); and
reports that vapor intrusion impacts generally have not been observed "at distances
greater than one or two houses beyond the estimated extent of the groundwater plume",
at sites where contaminated groundwater is the subsurface vapor source (Folkesetal.
2009).
However, we would note that vapor source types forwhich use of a 100-foot bufferwould
typically be inappropriate include:
Landfills where methane is generated in sufficient quantities to induce advective
transport in the vadose zone.107
Commercial or industrial settings where a vapor-forming chemical(s) has been released
within an enclosed space at a density that may result in significant advective transport of
the vapor(s) downward through cracks or openings in floors and into the vadose zone.
Leaking vapors from pressurized gas transmission lines.
In each of these cases, the diffusive transport of vapors may be overridden by advective
transport and the vapors may be transported in the vadose zone several hundred feet from the
source of contamination.108
Moreover, we would also note that anecdotal evidence indicates that in some settings buildings
greater than 100 feet from a plume "boundary" are affected by vapor intrusion, even when
106 Preferential migration routes are defined and discussed in Section 5.4. When present, they may facilitate
subsurfacevapor migration over distances greater than 100 feet.
107 EPA has also published Guidance for Evaluating Landfill Gas Emissions from Closed or Abandoned Facilities
(EPA 2005), w hich provides procedures and a set of tools for evaluating landfill gas emissions to ambient air and soil
gas migration due to pressure gradients.
108
For example, Little et al (1992) describe a landfill in southern California, where methane was detected in enclosed
spaces in nearby homes at concentrations approaching 1% by volume and chlorinated hydrocarbons had migrated
into a house 180 meters from the landfill.
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diffusion is the presumed mechanism of vapor migration.109 Furthermore, the presence of
conduits (e.g., sewer and drain lines that interceptand carry subsurface contamination
(Vroblesky et al. 2011), as well as permeable bedding for sewer lines or other utilities) or
preferential hydrogeologic pathways that facilitate unattenuated vapor migration in the vadose
zone, and otherfactors (e.g., presence of extensive surface covers, uncertainties in delineating
the boundaries) may extend the recommended inclusion distance for a vapor intrusion
investigation. For these reasons, EPA recommends investigating soil vapor migration distance
on a site-specific basis. That is, larger or smaller distances may need to be considered when
developing objectivesfor detailed vapor intrusion investigations and interpreting the resulting
data. Data from sub-slab and exterior soil gas sampling (see, for example, Sections 6.4.3, and
6.4.4, respectively)110 and indoor air testing (see, for example, Sections 6.3.4 and 6.4.1) can be
collected and evaluated to delineate or confirm areas at specific sites within which buildings are
potentially subject to vapor intrusion.
Criteria for Establishing "Boundaries" of the Plumes that Contain Vapor-forming Chemicals. This
Technical Guide is intended to be applied to existing groundwater plumes as they are currently
defined (e.g., Maximum Contaminant Levels, state standards, or risk-based concentrations).
However, it is important to recognize that some non-potable aquifers may have plumes that
have been defined by threshold concentrations significantly higher than drinking-water
concentrations. In these cases, contamination that is not technically considered part of the
plume may still have the potential to pose unacceptable human health risk via the vapor
intrusion pathway. Consequently, the plume definition may need to be expanded for purposes of
defining an inclusion zone for a vapor intrusion investigation. When groundwater is the
subsurface vapor source, EPA generally recommends comparing groundwater concentrations
to the VISLs to estimate the boundaries of the plume, when contaminated groundwater is a
subsurface vapor source, for purposes of establishing the boundaries of the vapor intrusion
inclusion zone.
Criteria for Establishing "Boundaries" of NAPL Plumes that Contain Vapor-forming Chemicals.
EPA generally recommends comparing soil gas concentrations to the respective VISLs to
estimate the boundaries of the vapor plume, when residual or free-phase NAPL is a subsurface
vapor source, for purposes of establishing the boundaries of the vapor intrusion inclusion zone.
6.2.2 Prioritizing Investigations with Multiple Buildings
At sites where numerous buildings are potentially subject to vapor intrusion (e.g., developed
areas with an extensive plume of contaminated groundwater), it may not be feasible or practical
at the outset to sample indoor air in each building or soil gas underneath or near each building.
In such circumstances, EPA generally recommends a "worst first" approach to prioritize
1DQ
Among other possibilities, vapor intrusion impacts observed to occur at distances greater than 100 feet in the
absence of a preferential migration route(s) may reflect imprecision in the interpolated edge of a plume, based upon
sampling data from sparse monitoring wells, and/or use of screening levels for drinking water, rather than for vapor
intrusion (i.e., VISLs), to delineate a plume's extent.
110 For assessing the extent of soil gas migration from the subsurface vapor source, EPA generally recommends
measuring soil gas concentrations, either sub-slab soil gas (preferably) or exterior soil gas, with a sufficient density to
characterize and understand spatial variability (Section 6.3.2).
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buildings for investigation. Factors that, if known, may warrant consideration in prioritizing
buildings for investigation include:
Source strength and proximity. Buildings overlying and near a source of vapors in the
vadose zone would generally be expected to have a greater potential for vapor intrusion
than buildings that do not overlie this same vapor source. Where the subsurface vapor
source is groundwater, buildings located over higher concentrations or shallower water
levels would generally be expected to have a greater potential for vapor intrusion than
buildings located over lower concentrations and deepergroundwater plumes.
Building types and conditions. Buildings that are continuously occupied may pose a
more immediate concern than buildings that are not currently occupied, if all other
factors (e.g., source strength and proximity) are equivalent. Nonresidential buildings with
bay-style doors that are routinely open may be better ventilated than othertypes of
nonresidential buildings, providing greater potential for dilution of vapor-forming
chemicals that enter the building via vapor intrusion.
Vapor migration ease. Buildings overlying vadose zones comprised of coarse geological
materials (e.g., gravel, boulders) generally would be expected to have a greater potential
for vapor intrusion than buildings overlying vadose zones comprised of fine-grained
materials (e.g., silts, clays), provided significant preferential migration routes (e.g.,
geologic fractures, utility corridors) are not present in the fine-grained layers.
Interviews and building surveys during development of the investigation work plan (or during the
preliminary analysis - see Section 5) also can provide useful information for prioritizing
buildings, when phased testing is chosen or indicated. Sections 6.3 and 6.4 provide additional
examples of survey information that can support planning, in addition to supporting data
interpretation.
In situations where "higher-priority" buildings and locations are investigated initially, investigation
of locations of other buildings may still be warranted, for example, to ensure that the CSM is
complete and accurate and that variability in the subsurface conditions and building conditions
is understood. There usually is substantial spatial variability in the concentrations of subsurface
vapors, caused by heterogeneities in the subsurface materials and otherfactors, that can result
in variability among buildings in vaporfluxand indoor air concentrations arising from vapor
intrusion. Additionally, building construction, building age and maintenance, and occupants'
activities that affect soil gas entry and air exchange rate will vary from building to building,
further adding to the variability in indoor air concentrations between buildings. Therefore, it may
be difficult to identify a priori either a "representative" or "reasonable worst case"111 building or
group of buildings, when it is determined that sampling all buildings is not practical.
For purposes of this Technical Guide, "reasonable worst case" is intended to be a semi-quantitative phrase,
referring to the upper portion of the high end of the exposure distribution, but less than the absolute maximum
exposure (see Glossary). Because EPA generally recommends a "worst first" approach to prioritize buildings for
investigation of the vapor intrusion pathway, "reasonable worst case" buildings wouldwarranta "higher priority" than
"representative" or typical buildings.
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When sampling all buildings is not practical, but other lines of evidence suggest that vapor
intrusion may be occurring, the site management team may wish to consider installing
engineered exposure controls for vapor intrusion mitigation in buildings without baseline indoor
air data (i.e., building mitigation as an early action - see Sections 3.3 and 7.8).
6.2.3 Planning for Community Involvement
Community involvement is an important component of any vapor intrusion investigation. EPA
recommends that a community involvement or public participation plan (see Section 9.1) be
developed or refined while planning a vapor intrusion investigation. Proper and sustained
community outreach and engagement efforts are critical to effectively implementing work plans
for vapor intrusion investigations, particularly when they involve sampling in a home or
workplace or on private property. Resuming and conducting community involvement at legacy
sites (i.e., sites that have a past history of agency involvement; see Section 9.6) can be
particularly complex The site planning team is encouraged to consult with appropriate EPA
colleagues experienced in community outreach and involvement efforts and utilize available
EPA planning resources, including those discussed in Section 9.
6.3 Characterize the Vapor Intrusion Pathway
As discussed in Section 2, the vapor intrusion pathway entails emanation of volatile chemicals
from a subsurface source(s) in a vapor form that migrates in the vadose zone, gradually
increases in amount underneath buildings as time passes, and enters buildings through
openings and conduits. As a result, detailed vapor intrusion investigations designed to develop
or enhance the CSM for a specific site will typically address one or more of the following
objectives, often in phases:112
Characterize the nature and extent of subsurface sources of vapors.
Characterize the subsurface migration paths between vapor sources and buildings
(potential "receptors").
Assess building(s) for their susceptibility to soil gas entry.
Evaluate the presence and concentration of a site-related subsurface contaminants) in
indoor air.
Identify and evaluate contributions of indoor and ambient air sources to concentrations
of hazardous vapors in indoor air.
112
The order of presentation is not intended to convey a suggested sequencing of objectives; rather, it follows the
presentation of the conceptual model of vapor intrusion in Section 2.
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These objectives are described in the following subsections for purposes of identifying the
primary lines of evidence typically developed and evaluated for each objective and describing
how the objectives fit together in developing and enhandng the CSM for a specific site an
characterizing vapor intrusion potential. This information is provided to assist the site planning
team in selecting and sequencing objectives for vapor intrusion investigations.
6.3.1 Characterize Nature and Extent of Subsurface Vapor Sources
Where the preliminary analysis indicates that subsurface contamination with vapor-forming
chemicals may be underlying or near buildings, EPA recommends that the nature and extent of
such contamination be well characterized. Source characterization data are critical to
developing a sound CSM and supporting confident, final decisions about the vapor intrusion
pathway.
Investigations to characterize the nature and delineate the extent of potential sources of vapors
may rely upon the results of groundwater sampling, soil sampling, or soil gas sampling, as
dictated by the site-specific source(s) and subsurface conditions.
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Groundwater Sources:
Where contaminated groundwater is a vapor source located near buildings, EPA
recommends that groundwaterobservation wells (i.e., monitoring wells) be installed at
strategic locations and used to assess groundwater flow and contaminant concentrations;
i.e., verify the nature and extent of groundwatercontamination through groundwater
sampling and analysis.113 Groundwater samples obtained from the uppermost portion of the
aquifer114 that underlies the study area of interest (i.e., where buildings are located) are
recommended for characterizing representative vaporsource concentrations for vapor
intrusion assessment. Forthis purpose, wells (or multi-level samplers) that are screened
across the water table interface are preferred and EPA recommends samples be collected
as close as possible to the top of the water table using approved sampling methods
designed to minimize loss of volatiles while sampling (EPA 2002a, EPA-ERT 2001 a).115
Ideally, the plume can be shown as stable or shrinking (i.e., is not migrating or rising in
concentration, including hazardous byproducts of any biodegradation), through multiple
rounds of sampling, so that vapor source concentrations can be confidently evaluated under
reasonably expected future, as well as current, conditions. Otherwise, the inclusion zone for
vapor intrusion (see Section 6.2.1) may expand over time and/or current sample
concentrations in or beneath a given building may under-estimate the reasonable maximum
vapor intrusion condition in the future.
For purposes of assessing vapor intrusion for specific buildings, groundwater samples from
wells nearer to buildings are generally recommended overthose from more distant wells.
Interpolation of the results obtained from two or more wells in the uppermost portion of the
aquifer may be warranted for these purposes when the spatial pattern suggests significant
lateral gradients in contaminant concentrations within the area of interest. However, for
purposes of determining whether contaminated groundwater poses acceptable human
health risk from vapor intrusion on an area-wide basis, it may be more appropriate to utilize
sampling results for the most greatly impacted well within the area of interest.
In addition, EPA generally recommends that a soil gas sample be collected immediately
above the groundwater table (and above the capillary fringe) (i.e., "near-source" soil gas
sample)116 to help characterize the subsurface vapor source. The results of such "near
113
Although a soil gas survey can also be employed as a screening tool to assist with the delineation ofaplume of
contaminated groundwater, EPA recommends that plume delineation ultimately be supported by the collection and
analysis of confirmatory groundwater samples at appropriate locations.
EPA recommends that, to the extent practical, groundwater samples be collected over a narrow interval (e.g., a
few feet or less) just below the water table when the data are to be used for assessing the potential for vapor
intrusion. Of course, the broader objectives of a site characterization will generally necessitate installation and
sampling of additional wells, from other depth intervals, to accurately characterize the full nature and extent of
groundwater contamination. Such wells and the broader topic of site characterization are not discussed in this
Technical Guide, which is focused instead on recommended guidelines that are pertinent specifically to vapor
intrusion.
115 If available groundwater data do not meet these criteria, the site data review team may consider whether they are
nevertheless representative of potential vapor source concentrations emanating from groundwater.
116 In this context and for purposes of this Technical Guide, "near" means "within a practically short distance." Site-
and location-specific circumstances and project-specific objectives typically will influence the quantitative definition of
"near" for purposes of collecting "near-source" soil gas samples.
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source" soil gas samples can be compared to calculations of the vapor concentration
expected when the soil gas is in equilibrium with the concentrations measured in shallow
groundwater (see AppendixC). Afavorable comparison (i.e., the two concentrations are
equivalent for each vapor-forming chemical in groundwater) would help to support the
results of the groundwater characterization. On the other hand, a "fresh water lens" (orother
site-specific conditions; see, for example, Section 6.3.2) could account for measured soil
gas concentration(s) being significantly lower than the calculated equilibrium
concentration(s).
Because fluctuations in water table elevation can lead to elevated vapor concentrations in
the vadose zone, EPA also recommends that "near source" soil gas sampling (and possibly
a soil gas survey) be considered in different seasons that coincide with groundwater
fluctuations.
Vadose Zone Sources:
Where contaminated soil or non-aqueous-phase liquid (NAPL) in the vadose zone is a
subsurface vaporsource, soil sampling using coring techniques for sample retrieval or using
sensors, such as a membrane interface probe, can be used to characterize the chemical
composition and general location of contamination; that is, bulk soil concentration data can
be used in a qualitative sense for this purpose. For example, high soil concentrations
generally would indicate impacted soil. Unfortunately, the converse is not always true. Non-
detect results for soil samples cannot be interpreted to indicate the absence of a subsurface
vapor source, because of the potential for vapor loss due to volatilization during soil
sampling, preservation, and chemical analysis.
Alternatively or in addition, a soil gas survey can be used to locate the primary source zone
and delineate the areal and vertical extent of the vapor-affected a re a. Generally, EPA
recommends that the soil gas survey include a soil gas sample collected immediately above
each contaminant source in the vadose zone (i.e., "near-source" soil gas samples) to help
characterize the vapor source.
Although a soil gas survey can generally be used to characterize many other subsurface vapor
sources (e.g., sewer and drain lines; landfills and other land-based disposal units;
impoundments and other land-based storage and/ortreatment units, pressurized tanks and
pipelines), additional approaches tailored to the specific source type may also warrant
consideration.
These sampling options are generally coupled with an understanding of the site-specific
subsurface conditions that control the location and extent of contamination (e.g., geologic
properties, including stratigraphy and level of heterogeneity; hydrogeologic conditions; sewers,
drains, and other conduits that lie underneath or intersect areas of groundwater and soil
contamination). Such understanding is generally developed by interpreting the data obtained
through borehole logging (i.e., visually inspecting soil cores and determining soil texture) or
geophysical tools.
EPA generally recommends sample locations be of sufficient density to adequately account for
spatial variability and heterogeneity in subsurface conditions. EPA generally recommends
consulting with individuals who have expertise in characterizing subsurface environmental
conditions (e.g., a geologist) when determining appropriate sampling locations and spacing.
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When combined with the data demonstrating that the property reflects the conditions and
assumptions of the generic model invoked in the VISLs (see Section 6.5.2), groundwater and
"near-source" soil gas samples can be compared to medium-specificscreening levels to
develop an initial quantitative perspective about the potential level of exposure and human
health risk posed by vapor intrusion. Section 6.5 provides additional information about risk-
based screening of vaporsourceconcentrations.
6.3.2 Characterize Vapor Migration in the Vadose Zone
As described in Section 2, geologic, hydrologic, biochemical factors in the vadose zone, as well
as elapsed time since the environmental release, can influence vapor migration and attenuation
in soil gas concentrations between subsurface vapor sources and nearby building(s).117 As
noted in Section 5.4, EPA recommends the CSM portray the current understanding of these
vadose zone conditions in and around the subsurface vapor source(s) and nearby building(s).
Furthermore, EPA recommends the CSM identify known or suspected preferential migration
routes that could facilitate vapor migration to greaterdistances and at higherconcentrations
than otherwise expected. When these conditions are not well established from existing
information, EPA recommends that a detailed investigation be scoped to address these data
gaps.
When combined with other data, as discussed further in Section 7.3, information about
subsurface vapor migration can support determinations that the vapor intrusion pathway is
complete under current conditions or may be complete under future conditions. In some cases,
vadose zone conditions may impose sufficient resistance to vapor migration to make the vapor
intrusion pathway insignificant. In these circumstances, information about subsurface vapor
migration, combined with other lines of evidence, can support determinations that the vapor
intrusion pathway is incomplete under current conditions, as discussed further in Section 7.3.
Investigations seeking to characterize vapor migration in the vadose zone generally entail, at a
minimum, a soil gas survey. Because soil gas concentrations can exhibit considerable spatial
variability, due to a variety of factors,118 EPA generally recommends that soil gas surveys collect
soil gas samples at multiple locations and depth intervals between the vaporsource and
building(s) (potential "receptors"). As a result, the soil gas survey may include samples collected
immediately outside the building ("exterior soil gas") at various depths or several depth intervals,
117 The horizontal and vertical distance over which vapors may migrate in the subsurface depends on the source
concentration, source depth, soil matrix properties (e.g., porosity and moisture content), and time since the release
occurred. For example, months or years of volatilization and vapor migration may be required to fully develop vapor
distributions in the vadose zone at sites with deep vapor sources or with impedances to vapor migration arising from
hydrologic or geologic conditions (Section 2.5; EPA 2012b). Under such circumstances, soil gas surveys conducted
soon after an environmental release may not yield data indicating the maximum extent of vapor migration.
118
Modeling of idealized scenarios provides additional demonstrations about spatial variability of soil gas
concentrations. For example, vertical profiles of soil gas concentration(s) can be very different underneath buildings
compared to locations exterior to the building and soil gas concentrations may not be uniform laterally, particularly in
the vicinity of the building, even when the vapor source is a laterally extensive plume of contaminated groundwater
(EPA 2012b). These simulation results indicate why EPA recommends that soil gas generally be sampled in multiple
sampling locations, when assessing subsurface vapor migration routes.
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as well as immediately beneath it (e.g., sub-slab soil gas sampling).119 If any shallow soil gas
samples are collected, EPA recommends they be collected as close as possible to the building
and at depths belowthe respective building foundation and no less than five feet belowground
surface, depending on site-specific conditions. Where crawl spaces are present, crawl space air
sampling may also be conducted.
Generally, EPA recommends that the soil gas survey include a "near-source" soil gas sample
collected immediately above each source of contamination to help characterize the subsurface
vapor source (see Section 6.3.1). The results of such "near source" soil gas samples can be
compared to calculations of the vapor concentration expected when the soil gas is in equilibrium
with the concentrations measured in shallowgroundwater (see AppendixC), when the
subsurface vapor source is in the groundwater. Geologic, hydrologic, or biologic impedances to
vapor migration may be indicated if the measured "near source" soil gas concentrations are
significantly lower than the calculated equilibrium concentrations.
To characterize subsurface migration in the vadose zone, soil gas survey data are generally
coupled with an understanding of the site-specific subsurface conditions that influence vapor
migration and attenuation (e.g., geologic properties, including stratigraphy and level of
heterogeneity; hydrologic conditions, including groundwater elevation and soil moisture; 12ฐ and
biological properties, including availability of oxygen to support aerobic biodegradation).121 Such
geologic understanding is generally developed by interpreting the data obtained through
borehole logging and geophysical tools. Soil permeability to airflow can be measured in the
field (McHugh et al. 2013) and would be used to corroborate inferences based upon borehole
logging data. Hydrologic conditions can be characterized by analyzing soil samples for porosity
and moisture content and by hydrologic modeling. For potentially biodegradable contaminants,
an intensive soil gas survey to establish current vertical profiles for contaminant vapors and
oxygen (and, in some cases, biodegradation products, such as carbon dioxide or methane)122
may be able to demonstrate that biodegradation is responsible for attenuating vapor migration
11Q
EPA recommends that spacing of soil gas sampling locations generally consider the extent and location of the
subsurfacevapor source, distance between the building and the source, and other site-specific factors.
120
Tillman and Weaver (2007) conducted hydrologic modeling and collected field data, which show edthat moisture
content determined from soil cores taken external to a building may over-estimate soil moisture underneath the
building. They inferred that vapor intrusion assessments based upon moisture content in soil from open areas
between buildings may under-estimate vapor intrusion potential.
191
As noted in Section 2, vapor migration in the vadose zone can be impeded by several factors, including soil
moisture, low-permeability (generally fine-grained) soils, and biodegradation. Significant characterization of the
vadose zone may be needed to demonstrate that such geologic, hydrologic, and biologic features are laterally
extensive over distances that are large compared to the footprint of the building and the extent of the subsurface
vapor source at a specific site.
1??
Interpretation of profiles for carbon dioxide and methane can be challenging, due to the presence of natural
sources unrelated to contaminant biodegradation (Holden and Fierer 2005).
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to a greater extent than can be attributed to advection and diffusion in the vadose zone.123'124
For both purposes, samples collected directly underneath the building will tend to be more
representative of conditions influencing vapor intrusion potential than samples collected outside
the building footprint, all else being equal.
When conducted contemporaneously for multiple buildings, a soil gas survey and
characterization of the vadose zone can help identify distances from subsurface vapor sources
beyond which threats from vapor intrusion are not reasonably expected, as mentioned in
Section 6.2.1. At sites with a limited number of potentially affected buildings, it may be feasible
to characterize the subsurface vapor migration near and surrounding all of them. However, at
sites where a large number of buildings may be affected, this approach is not likely to be
feasible; in these cases, EPA generally recommends that the site manager seek the advice of
an individual familiar with the site-specific subsurface conditions (typically a geologist) to help
guide selection of appropriate sampling locations and assess whether "representative" or
"reasonable worst case" locations can be identified, as appropriate to the objectives of the
investigation. Because there usually is substantial spatial variability in the concentrations of
subsurface vapors, caused partially by heterogeneities in the subsurface materials, it may be
difficult to identify a priori locations that are either "representative" or are "reasonable worst
case" subsurface conditions.
Subsurface investigations of vapor intrusion also generally warrant an evaluation of utility
corridors, which can facilitate unattenuated vapor transport over longer-than-anticipated
distances and/or vapor migration towards and into buildings that are serviced by the utility. EPA
also recommends subsurface investigations of vapor intrusion consider whether sewers and
other man-made conduits have the potential to transport NAPLs, contaminated groundwater,
and/or vapors (through soil) towards and/or directly into buildings. Public and facility records
may be useful sources of information about utility and sewer locations, which may provide
maps, "as built diagrams," or construction specifications. Depending upon the CSM, sampling of
vapors within the utility corridor (or within a sewer, if present) may be warranted to characterize
vapor migration in the subsurface (or characterize a secondary source of vapors - see Sections
6.3.1 and Section 2.1).
Reasonably expected future risks posed by the subsurface vapor source(s) warrant
consideration, in addition to risks posed under current conditions, "in order to demonstrate that a
site does not present an unacceptable risk to human health and the environment" (EPA 1991 a).
For example, when evaluating subsurface vapor migration and attenuation in locations where
buildings do not exist, it is important to recognize that the conditions in the vadose zone and soil
123
At sites w here aerobic biodegradation is limiting the upward migration of petroleum hydrocarbon vapors, for
example, the vertical concentration profile will typically show higher concentrations of petroleum hydrocarbons and
lower (or non-detect) concentrations of oxygen in deeper soil gas samples. Atthese same sites, the vertical
concentration profile will typically show lower (or non-detect) concentrations of petroleum hydrocarbons and higher
concentrations of oxygen in shallower soil gas samples. Because weather events can affect rates of oxygen
replenishment in the vadose zone (Lundegard et al. 2008), multiple rounds of such sampling are recommended to
demonstrate that biodegradation consistently poses a significant impedance to upward vapor migration. This
recommendation is particularly apt wherethe subsurface vapor source is strong (e.g., unweathered NAPL in the
vadose zone) relative to time-variable processes supplying oxygen to the vadose zone.
In this context, mathematical modeling (see Section 6.6) can be employed to characterize vapor migration
attributable to advection and diffusion in the vadose zone.
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gas concentrations may be changed as a result of constructing a new building and/or supporting
infrastructure. The moisture content may decrease and the moisture profile change in the
vadose zone as a result of reduced infiltration of rainwater below a building footprint (Tilman and
Weaver 2007). The permeability to vaporflowin the vadose zone may be altered in the
foundation vicinity due to construction. Finally, the future presence of extensive surface covers
and/or utility corridors may also modify the vertical and horizontal profile of soil gas
concentrations in the subsurface (EPA 2012b). As a result, EPA recommends that appropriate
lines of evidence in addition to a soil gas survey (e.g., mathematical modeling, where
parameters are chosen to represent conditions that give a high-impact case - Section 6.6) be
developed and considered to support any determination that a future building will not be subject
to vapor intrusion or will not pose unacceptable human health risk for occupants. Owing to the
potentially unpredictable plans for building construction and site redevelopment, as well as
potentially unpredictable changes in the transitory soil characteristics (e.g., soil moisture) and
soil gas concentrations, institutional controls may be warranted (e.g., to inform the need for a
confirmatory evaluation of the vapor intrusion pathway) when new buildings are constructed in
areas where the subsurface vapor source(s) has(have) significant potential to pose a vapor
intrusion threat.
6.3.3 Assess Building Susceptibility to Soil Gas Entry
When elevated concentrations of vapor-forming chemicals accumulate in the soil gas
immediately underneath the foundation, surrounding the basement, or within the crawl space of
a vulnerable building, then soil gas entry (i.e., vapor intrusion) can lead to unacceptable levels
of subsurface contaminants in indoor air, depending upon building- and site-specific
circumstances. As discussed in Section 2.3, soil gas can enter a building when openings are
present and driving 'forces' exist to drawthe vapors from the subsurface through the openings
into the indoor environment.
Single-family detached homes can generally be presumed to have openingsfor soil gas entry;
as such, they will generally be susceptible to soil gas entry unless a mitigation system (e.g.,
radon mitigation system) is present and operating as intended. Some buildings are more
susceptible to soil gas entry than others. For example, buildingswith significant openings, such
as:
buildings with deteriorating basements or dirt floors, which generally provide poor
barriers to vapor (soil gas) entry; and
buildings with sumps (or other openings to the subsurface) that can facilitate transport of
vapors via soil gas entry.
EPA recommends that appropriate lines of evidence be employed to assess susceptibility to soil
gas entry, when this objective is selected as part of a site-specific investigation plan for vapor
intrusion assessment. Vulnerability to soil gas entry can be assessed for a specific building by
using any of several methods, including:
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Concurrently monitoring indoor air samples for presence of radon and finding radon in
indoor air at levels greater than in ambient air.125
Concurrently monitoring indoor air and ambient air (see Section 6.3.5) and finding cis-
1,2-dichloroethylene, vinyl chloride, 1,1 -dichloroethylene, or 1,1 -dichloroethane in indoor
air at levels greater than in ambient air, when and where they are present in the
subsurface vaporsource(s), but are not used indoors.126
Employing a photoionization detector (PID) or other real-time in-field device, capable of
detecting parts per billion by volume (ppbv) levels, to directly survey suspected locations
of soil gas entry (e.g., utility penetrations, sumps) and finding elevated readings of
vapors.
Conducting a visual inspection for cracks and holes in concrete foundation slabs,
basement walls, or any floor drain(s). (Openings for soil gas entry will not necessarily be
visible or accessible for inspection, so the absence of visible openings, by itself, is
insufficient to demonstrate that a building is not susceptible to soil gas entry.)
Monitoring pressure differences between the building and subsurface environment to
characterize the 'driving force' for soil gas entry and the effects of the heating,
ventilation, and air-conditioning (HVAC) systems.
Injecting tracers into the subsurface at selected concentrations and subsequently finding
it in indoor air samples.
Certain complementary information obtained forthe building, as identified in Section 6.4.1, can
also support such assessments. Relevant information includes the operating characteristics of
HVAC systems.
In many commercial buildings, the HVAC system brings outdoor air into the building, potentially
creating building over-pressurization relative to the outdoor environment. When the building is
125
Because vapor intrusion and radon intrusion entail similar mechanisms for subsurfacevapor migration and gas
entry into buildings and structures (Section 2.3), naturally occurring radon may serve as a tracer to help identify those
buildings that are more susceptible to soil gas entry than others. Buildings with radon concentrations greater than
levels in ambient air are likely susceptible to soil gas intrusion and would likely be susceptible to intrusion of any
chemical vapors in the subsurface. On the other hand, the radon concentration in a building is not generally expected
to be a good quantitative indicator of indoor air exposure concentrations of vapor-forming chemicals arising from sub-
surf ace contamination. Hence, radon measurement is not generally recommended as a proxy for directly measuring
vapor-forming chemicals in indoor air. Among other factors, the distribution of radon-emanating rock and soil and the
spatial and temporal variability of their source strength are generally expected to be very different (e.g., tending to be
broader and more uniform) than the distribution and source strength variability for subsurface sources of chemical
vapors.
126 EPA (2011 a) reports that "vinyl chloride, 1,1-dichoroethylene, cis-1,2-dichloroethylene, and 1,1-dichloroethane are
rarely detected in background indoor air." DoN (2011 a) also reports that vinyl chloride and cis-1,2-dichloroethylene
"are rarely detected in background indoor air." When they are subsurface contaminants, volatile chemicals that are
rarely or never present in indoor sources can be inferred to arise in indoor air via vapor intrusion "withoutf urther
explanation" (DoN 2011a). Brenner (2010), for example, employed this principle (and cis-1,2-dichloroethylene) to
identify buildings susceptible to vapor intrusion and to diagnose the relative contributions of vapor intrusion and
infiltration to indoor air concentrations.
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over-pressurized sufficiently to eliminate the driving force for soil gas entry over at least a
portion of the building foundation, vapor intrusion potential is diminished.127
Reasonably expected future risks posed by subsurface contamination warrant consideration, in
addition to risks posed under current conditions, "in order to demonstrate that a site does not
present an unacceptable risk to human health and the environment' (EPA 1991 a). For example,
current building use and HVAC systems might not be sustained perpetually. Therefore, when
the subsurface vaporsource(s) underneath or neara building with an over-pressurizing HVAC
system has(have) significant potential to pose a vapor intrusion threat, it may be useful to
assess susceptibility to soil gas entry and diagnose vapor intrusion (also see Sections 6.3.4 and
6.4.1) in such buildings under conditions when the HVAC system is not operating. (In addition,
indoor air testing could be conducted during periods when the HVAC system operates with
diminished flows, such as weekends or evenings.) The results of such testing can be used to
support determinations about whether the vapor intrusion pathway is "potentially complete" and
is reasonably expected to pose unacceptable human health risk (see Section 7.4) in the
future,128 in which case a response action(s) may be warranted (see Section 7.7). Forexample,
if the results indicate susceptibility to soil gas entry when the HVAC system is not in operation
and vapor intrusion under these conditions has the potential to pose a health concern, then the
building may warrant future monitoring (e.g., continuous monitoring of the pressure gradient
across the foundation or indoor air testing) and/orengineered exposure controls, which may be
enforceable through an institutional control (1C) (see Section 8.6).
Likewise, well-designed and operated radon mitigation systems generally should diminish vapor
intrusion via soil gas entry under current conditions. Therefore, buildings with pre-existing radon
mitigation systems, which overlie or are near subsurface vapor sources, could be tested under
conditions where the radon mitigation system is temporarily not operated to support decisions
about monitoring and ICs as part of a vapor intrusion remedy.129
6.3.4 Evaluate Presence and Concentration of Subsurface Contaminants in Indoor Air
Indoor air sampling (see Section 6.4.1) using time-integrated sampling methods or grab
samples can confirm the presence, if any, of a site-related vapor-forming chemical (i.e., one
comprising the subsurface vapor source(s)) in the indoor environment. When combined with
data characterizing subsurface vapor migration and demonstrating the building is (or is not)
susceptible to soil gas entry, indoor air sampling data can support determinations that the vapor
intrusion pathway is (or is not) complete for a given building, as discussed furtherin Section 7.3.
When conducted contemporaneously in multiple buildings, indoor air sampling can, in concert
with soil gas survey data and data delineating subsurface vapor sources, help identify the
boundaries of the land area(s) within which buildings are known or suspected to have indoor air
concentrations of subsurface contaminants arising from vapor intrusion (also see Section 6.2.1).
127
Over-pressurization may not be uniform throughout a building, particularly in large buildings. Over-pressurization
in portions of a building will not necessarily mitigate all openings for soil gas entry.
"Both current and reasonably likely future risks need to be considered in order to demonstrate that a site does not
present an unacceptable risk to human health and the environment." (EPA 1991a).
EPA recommends that state and local laws be researched before any such testing is conducted. Some areas have
local ordinances governing operation and maintenance of radon mitigation systems.
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Indoor air sampling is most commonly conducted using time-integrated sampling methods,
when characterizing exposure concentrations for building occupants (see Section 6.4.1), which
may include contributions from "indoor" or ambient air sources of these chemicals (see Section
2.7). For example, time-integrated concentrations of hazardous vapors in samples of indoor air
can be compared to appropriate, risk-based screening criteria (see Section 6.5) to obtain some
preliminary insights about the potential level of exposure and risk posed by vapor intrusion or
can be used to support a human health risk assessment (Section 7.4) about vapor-forming
chemicals found in the subsurface environment. 13ฐ
When sampling indoor air (or sub-slab soil gas) to characterize exposure concentrations arising
from vapor intrusion, EPA recommends removing potential indoorsources of vapor-forming
chemicals (see Section 2.7 and 6.4.1) from the building to strive to ensure that the
concentrations measured in the indoor air samples are attributable to the vapor intrusion
pathway. However, even after removing indoorsources, their effects may linger depending on
source strength, relative humidity in the building, the extent to which the contaminants have
been absorbed by carpets and other fabrics or "sinks," and air exchange rate. In addition, field
experience suggests that it may not be possible to remove all indoor sources. It may be
particularly impractical to do so in industrial settings where vapor-forming materials are used or
stored.
6.3.5 Identify and Evaluate Contributions from Indoor and Ambient Air Sources
As noted in Section 2.7 herein, indoorair is likely to contain detectable levels of a number of
vapor-forming chemicals regardless of whether the building overlies a subsurface vapor source,
because indoor air can be impacted by a variety of indoor and outdoor vapor sources unrelated
to site contamination. The contribution of indoor and outdoor vapor sources (or both) to indoor
air concentrations is referred to as "background" throughout this Technical Guide, when they do
not arise from site-related contamination (see Glossary). Information on 'background'
contributions of site-related, vapor-forming chemicals in indoor air is important to risk managers
because generally EPA does not clean up to concentrations below natural or anthropogenic
background levels (EPA2002e).
To determine if a subsurface vapor source(s) is (or are) responsible for indoor air contamination,
EPA recommends that such background sources of site-specific analytes be identified and
distinguished from vapor-forming chemicals arising from vapor intrusion. A comprehensive
investigation of all background substances found in the environment is usually not
recommended. For example, sub-slab soil gas and ambient air samples typically would not be
analyzed for radon for purposes of characterizing 'background' exposures perse, whereas EPA
would recommend analyzing for radon if its precursorwas part of a regulated release to the
subsurface environment (EPA 2002e).131 Generally, EPA recommends the site planning and
nn
In certain cases, depending in part on the results (e.g., concentrations exceed risk-based screening levels), indoor
air sampling data may be a sufficient basis for supporting decisions to undertake pre-emptive mitigation/early action
(see Sections 3.3 and 7.8) in lieu of additional rounds of sampling and analysis or an evaluation of the contribution of
background sources to indoor air concentrations.
131
Sub-slab soil gas and indoor air samples might also be analyzed for radon, where its precursorwas not part of a
release to the subsurface environment, for purposes of diagnosing vulnerability to soil gas entry (see, for example,
Section 6.3.3), depending upon the objectives of the vapor intrusion investigation.
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data evaluation team limit chemical analyses to those vapor-forming chemicals known (based
upon subsurface contaminant characterization) or reasonably expected (based upon site
history) to be present as a result of a release to the subsurface environment.
To support evaluations of sources of indoor air concentrations, EPA recommends conducting a
building survey (see Section 6.4.1) that identifies in individual buildings known or suspected
indoor sources of the vapor-forming chemicals also found in the subsurface (see Section 2.7)
and characterizing ambient air quality (see Section 6.4.2) in the site vicinity for these same
chemicals. Key supporting information includes: (1) the locations and types of known or
potential indoor vapor sources; (2) information about outdoor vapor sources related to the site
(e.g., locations of chemical storage, use, and/or release to the environment); (3) information
about outdoor vapor sources un-related to the site, such as nearby commercial or industrial
facilities and mobile sources (e.g., cars, trucks, and other equipment); and (4) data on the local
ambient air quality.
Interviews of building occupants and inspections of buildings can be helpful initial sources of
information about indoor uses and storage of vapor-forming consumer and commercial
products.132 In addition, vapor-detecting field instruments and in-field gas chromatographs133
can be used to locate indoor vapor sources. Grab (essentially short-duration) samples of indoor
air, as described in Section 6.4.1, can be useful for identifying spedfic vapor-forming chemicals
emanating from indoor vapor sources of consumeror commercial products. When the objective
is to quantitatively distinguish contributions to indoorair concentrations from vapor intrusion
versus contributions from indoor and ambient air sources, as described below, EPA
recommends obtaining indoorair concentrations using time-integrated sampling methods (see
Section 6.4.1) instead of grab samples.
If the subsurface vapor source(s) is (or are) comprised of multiple vapor-forming chemicals and
the subsurface source medium (e.g., soil, groundwater) and location are identical for these
chemicals, then contemporaneous samples of sub-slab soil gas (see Section 6.4.3) and indoor
air (see Section 6.4.1) can be compared, potentially supporting one of the following conclusions:
Results indicating vapor intrusion as solely responsible for vapor concentrations in indoor
air. The predominant vapor-forming chemicals in the sub-slab soil gas and their relative
proportions in indoorair and sub-slab vaporsamples would be expected to be similar,
whereas their concentrations in sub-slab soil gas would be expected to be substantially
higher than in indoor air,134 if vapor intrusion is solely responsible for indoor air
132
Information about the chemical composition of commonly encountered products is provided by the U.S. Navy
(DoN 2011 a, Appendix A) in its guidance for background analysis for the vapor intrusion pathway.
133
Gorder and Dettenmaier (2011) reported on the use of a field-portable gas chromatograph and mass spectrometer
to identify specific sources of vapor-forming chemicals and estimate their mass emission rate(s). EPA's
Environmental Response Team has employed the Trace Atmospheric Gas Analyzer (TAGA) mobile laboratory for
similar purposes.
134
Based upon the generic sub-slab attenuation factor identified in Section 6.5.3 herein, sub-slab soil gas
concentrations can be expected to typically exceed indoor air concentrations by 33 times or more in residences that
are impacted by vapor intrusion (i.e., 33 is the inverse of an attenuation factor of 0.03), when background sources are
negligible and the building is under-pressurized relative to the subsurface during indoor air sampling.
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concentrations.135 If recalcitrant (i.e., not subject to biodegradation in the vadose zone), the
predominant vapor-forming chemicals and their relative proportions in the subsurface vapor
source would likewise tend to be similar to those in indoor air if vapor intrusion is solely
responsible for indoorair concentrations.136
Results indicating indoor vapor sources as primarily responsible for indoorair
concentrations. If a vapor-forming chemical is presentwith an elevated concentration in
indoor air, but is not present or is negligibly present in sub-slab soil gas samples (or
representative samples of the subsurface vapor source), then the presence of this
contaminant in indoor air may not arise from the vapor intrusion pathway, but rather from
indoor sources or other background sources (e.g., ambient air). In these circumstances,
EPA recommends considering additional attempts to identify and temporarily eliminate
indoor sources, where practical, and re-sample indoor air and sub-slab soil gas after doing
so.
Likewise, outdoor (ambient) air samples can be collected (see Section 6.4.2)
contemporaneously with indoor air (see Section 6.4.1) and sub-slab soil gas (see Section 6.4.3)
samples, as recommended in Section 6.4.
Results indicating outdoorvaporsources as primarily responsible for indoor air
concentrations. If a vapor-forming chemical(s) is(are) detected in outdoorairand indoorair
at similar concentrations, but is(are) not present in sub-slab soil gas samples (or
representative samples of the subsurface vapor source) or is present in the subsurface
samples at concentration(s) similar to indoor air),137 then the presence of this contaminants)
in indoor air may not arise from the vapor intrusion pathway, but rather from outdoor sources
(i.e., ambient air).
Concentrations of vapor-forming chemicals in indoor air, sub-slab soil gas, and ambient air can
be compared, as described above, using an individual time-integrated sample for each medium.
Recognizing that weather conditions and building operations can lead to variable contributions
from vapor intrusion and ambient air infiltration overtime, EPA recommends, however, that such
135
Conversely, if there is an interior source of a vapor-forming chemical in indoor air samples, the relative proportion
of this chemical in indoor air will be greater than its respective proportion in the sub-slab soil gas, even where vapor
intrusion is occurring, assuming that the other vapor-forming chemicals in the sub-slab soil gas do not have
'background' sources.
Conversely, if there is an interior source of a vapor-forming chemical in indoor air samples, the relative proportion
of this chemical in indoor air will be greater than its respective proportion in the subsurfacevapor source or in "near-
source" soil gas samples (see Section 6.3.1), even where vapor intrusion is occurring, assuming that the other vapor-
forming chemicals in the sub-surface do not have'background' sources.
137
Sample concentrations of vapor-forming chemicals in indoor air and sub-slab soil gas can be compared to
conservative, risk-based screening levels to provide a complementary line of evidence. Generally, vapor-forming
chemicals with concentrations that consistently fall below screening levels (see Section 6.5) through multiple
sampling events (see Section 6.4) warrant no further action or study, so long as the exposure assumptions match
those taken into account by the calculations and the site fulfills the conditions and assumptions of the generic
conceptual model underlying the screening levels (see Section 6.5.2).
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comparisons be made for multiple sets of paired samples, collected in different seasons,138 to
support any conclusion thatvapor intrusion is not a significant contributorto indoor air
concentrations, which can instead be attributed to indoor and outdoor sources unrelated to the
subject site. Even with a few sets of such samples, rigorous statistical tests may not be feasible.
Nevertheless, comparing contemporaneously measured concentrations and proportions of
vapor-forming chemicals in indoorair, subsurface media, and ambient air can be effective for
this investigation objective, particularly when one (or more) of the analytes is known to be
present only in the subsurface or in ambient air.
The following hypothetical example illustrates how site-specific sampling data might inform
conclusions about the relative contributions of indoorversus subsurface vapor sources in a
building overlying contaminated groundwater:
Example: Time-integrated samples of indoorair, outdoor air, and subslab soil gas were
collected contemporaneously for a building that overlies shallow groundwater that is
contaminated with a suite of vapor-forming chemicals (designated as VFCA, VFCB,
VFCC, and VFCD). The sampling results are summarized as follows:
Vapor-forming
Chemical in
Groundwater
VFCA
VFCB
VFCC
VFCD
Time-weighted Sample Concentrations (ug/mj)
Subslab Soil Gas
1
33,000
5,200
15,000
Indoor Air
0.65
26
5.8
15
Outdoor Air
0.75
0.18
0.14
0.51
Ratio of Subslab
Concentration to
Indoor Air
Concentration
3
1,300
900
1,000
Based upon the conceptual site model, the presence of these vapor-forming chemicals
in outdoor (ambient) air is believed to be due to anthropogenic sources that are not
associated with the environmental release responsible for the subsurface contamination.
The building is presumed to be susceptible to vapor intrusion, as indicated by pressure
monitoring data that indicate building under-pressun'zation, relative to the subsurface
environment, during the sampling event (as discussed further in Section 6.4.1).
Based upon these findings, the presence of VFCB, VFCC, and VFCD in indoorair
appears to be solely or primarily attributable to vapor intrusion. The relative proportions
138
A goal of collecting multiple samples is to observe and characterize a reasonable maximum vapor intrusion
condition for the respective building. Because weather conditions and building operations can lead to time-variable
contributions from vapor intrusion (e.g., driving forces for vapor intrusion; see Section 2.3) and ambient air infiltration
(see Sections 2.4), indoor air concentrations of vapor-forming chemicals can be expected to vary overtime. An
individual sample, collected at a randomly chosen time, may under-estimate or over-estimate average and
reasonable maximum exposure conditions (see Section 6.4.1) to different degrees, depending upon the season of
sample collection and other factors.
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of these subsurface contaminants in indoor air and sub-slab vapor samples are similar,
as indicated by a similar ratio ofsubslab to indoor air concentration, considering
analytical uncertainty. In addition, their indoor air concentrations exceed those found in
the paired sample of ambient air.
By contrast, the presence of VFCA in indoor air may be entirely attributable to infiltration
of ambient air, as the sample concentrations in indoor air and outdoor are similar,
considering analytical uncertainty.
Recommended next steps in the investigation might include a human health risk
assessment (see Section 7.4) and a reviewofthe conceptual site model (see Section
5.4) to evaluate whetherthe different conclusion for VFCA can be reasonably explained
(e.g., by vapor attenuation in the vadose zone that is expected to be substantially
greater than for VFCB, VFCC, and VFCD).
EPA has compiled and published an evaluation of studies pertaining to indoor air concentrations
of volatile organic compounds in North American residences in 1990-2005 (EPA 2011 a), which
can be employed to identify whether measured indoorair concentrations in residences exceed
the historical range of background concentrations. Specifically, if measured indoorair
concentrations are found to greatly exceed the historical range of background levels, there is a
greater likelihood thatthe indoorair concentrations are the result of vapor intrusion. This
conclusion is supported by the expectation that current levels of vapor-forming chemicals in
ambient air and in indoor air due to indoor and ambient air sources are likely to be lower than
those observed historically,139 due to regulations and business practices fostering less use of
toxic chemicals in consumer products and industrial processes and reduced emissions from
mobile and stationary sources. As a result of this expectation, EPA does not recommend the
use of generic values of historical background concentrations, even those cited in peer-
reviewed publications or available from databases maintained by regulatory agencies, to
characterize current levels in any building, for purposes of supporting condusions that indoor air
concentrations are due to'background' sources. Rather, EPA recommends that site-specific
data (e.g., sub-slab, indoor air and ambient air sampling data) be obtained (as described in
Sections 6.4.1, 6.4.2, and 6.4.3), and evaluated, as described above, when the investigation
objectives include determining whether indoor air concentrations arise from indoor or ambient
air sources.
The following additional approachesfor identifying and characterizing 'background' sources may
warrant consideration in special situations:
McHugh etal. (2012) have demonstrated the prindple that building over-pressurization
can be employed temporarily to minimize vapor intrusion and facilitate measuring indoor
-ion
McCarthy et al. (2007), for example, analyzed ambient air data for 25 toxic substances collected in the United
States from 1990 through 2005 and found that concentrations of many halogenated volatile organic compounds were
declining at most sites and evaluation periods (i.e., 1990-2005, 1995-2005, and 2000-2005). They found that
concentrations of petroleum hydrocarbons, such as benzene, associated with mobile sources were all consistently
decreasing over the three evaluation periods at most sites.
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air concentrations under conditions where only indoor sources may be contributing. 14ฐ At
this time, however, there are no standard practices for using over-pressurization to
assess 'background' contributions, which is a research and development need (SERDP-
ESTCP2014).
Forensic and multi-variate statistical methods have been described and illustrated by the
U.S. Navy (DoN 2011 a) in its guidance for background analysis for the vapor intrusion
pathway.
6.3.6 Select, Prioritize, and Sequence Investigation Objectives
Site-specific investigations of potential vapor intrusion frequently begin with pursuing one or
more of the objectives presented in Sections 6.3.1 through 6.3.5. Criteria potentially warranting
consideration by the site planning team when making decisions about prioritizing and
sequencing investigation objectives include, but are not limited to: site scenario (see Section
6.1); building occupants who may be particularly sensitive to the potentially toxic effects of
vapors; buildings that are more susceptible to soil gas entry (e.g., buildings with deteriorating
basements or dirt floors; whether there are any significant data gaps in the CSM (see Section
5.4); and relationships with and perspectives of the owners and occupants of potentially
impacted buildings.
Characterizing subsurface vapor sources (Section 6.3.1), characterizing subsurface vapor
migration (Section 6.3.2), and evaluating the presence of subsurface contaminants in indoor air
(Section 6.3.4) - are frequently candidates for an initial objective and each can be pursued
separately. For example, characterizing subsurface vapor sources (Section 6.3.1) may be a
useful initial choice when responding to an initial report abouta release of hazardous, vapor-
forming chemicals to the subsurface from a commercial or industrial operation or when buildings
do not exist currently, but are expected in the future. Characterizing subsurface vapor sources
may also be a useful initial choice when building owners or occupants are reluctant to grant
access for indoor air testing. In this situation, the site planning team may need to pursue
subsurface investigations more intensely to characterize vapor intrusion potential before being
granted building access. On the other hand, testing indoorair is recommended as an initial
objective when responding to reports of odors in buildings or clusters of inhalation-related
symptoms and there is credible information to suggest that a subsurface environmental release
may be a contributing factor (see Section 5.2).
In a different scenario, characterizing subsurface vapor migration (Section 6.3.2) may be a
useful starting point when addressing sources that are comprised of potentially biodegradable
chemicals or that are suspected to occur below an extensive geologic layerthat might impede
upward diffusive migration. For large buildingswith HVAC systems that may over-pressurize the
interior relative to the subsurface environment, EPA generally recommends: a building
assessment early in the investigation, which obtains and weighs the complementary information
140 Indoor air concentrations measured after sufficient periods of over-pressurization may be indicative of
'background' levels, whereas indoor air concentrations detected before and after a sufficient period of "rebound" from
temporary over-pressurization may be indicative of joint contributions from'background' sources and any vapor
intrusion from the subsurface.
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identified in Section 6.4.1, to support investigation planning; and an evaluation of susceptibility
to soil gas entry under conditions when the HVAC system is not operating (see Section 6.3.3).
Each of the investigation objectives described in Sections 6.3.1 through 6.3.5 may, in some
cases, be conducted itenatively with increasing complexity as the investigation proceeds and the
CSM is refined. For example, field instruments can be useful for locating potential background
sources (e.g., household or commercial cleaning products) (see Section 6.3.5) and grab
(essentially short-duration) samples of indoorair, as described in Section 6.4.1, can be useful
for characterizing the chemical composition of identified indoor sources of vapors during an
initial building reconnaissance while potential background sources are surveyed. These
activities might be followed by indoor air and sub-slab soil gas sampling, using time-integrated
sampling methods as described in Section 6.4.1 and 6.4.2, to distinguish subsurface
contributions from indoorsources. More advanced methods of distinguishing the various
potential contributions to indoorair might be utilized, if warranted, in intermediate phases of the
investigation undersuch an iterative approach.
6.4 General Principles and Recommendations for Sampling
Sampling of indoorair, outdoor air, soil gas, and groundwaterand analysis for vapor-forming
chemicals can play an important role in vapor intrusion investigations for one or more of the
objectives identified in Section 6.3. This subsection summarizes for indoor air, outdoor air, sub-
slab soil gas, exterior soil gas, and groundwater the following:
Principal methods for collecting samples.
Potential uses of the resulting sampling data.
Recommended practices for sample collection.
Unique or frequently encountered logistical issues.
We would note that soil and NAPL sampling has been and may be used to characterize the
nature (e.g., chemical composition) and general location of subsurface vapor sources (see
Section 6.3.1). Information about soil sampling can be found in Standard Operating Procedures,
Soil Sampling (EPA-ERT 2001 b). However, bulk soil (as opposed to soil gas) sampling and
analysis is not currently recommended for estimating the potential for vapor intrusion to pose
unacceptable human health risk in indoor air, because of the potential for vapor loss due to
volatilization during soil sampling, preservation, and chemical analysis. In addition, there are
uncertainties associated with soil partitioning calculations.
To ensure that the sampling data will meet the site-specific data quality needs, EPA
recommends that the sampling and analytical methods selected by the site planning team be
capable of obtaining reliable analytical detections of concentrations less than project-
appropriate, risk-based screening levels (e.g., VISLs). Towards that end, EPA recommends
that, as part of establishing site-specific data quality objectives (DQOs), the planning and data
collection team(s) consult with a laboratory skilled in the analysis of air and soil gas samples
and choose sampling and analytical methods capable of routinely attaining the desired detection
sensitivity for each medium.
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EPA also recommends that the site planning team identify and utilize appropriate sampling
locations and durations and address spatial and temporal variability to fulfill the specific
objectives of the investigation, which may include obtaining data to characterize the potential
human exposure in a building(s). EPA recommends the CSM, the objective(s) of the
investigation, and other site-specific information be considered in determining the number and
types of samples used at a specific site.
The sampling duration depends on the type of medium being sampled (for example, soil gas,
sub-slab soil gas, and indoor or outdoor air) and analytical methods (forexample, Method TO-
15). Some of the key recommended considerations are provided in the following subsections.
Several rounds of sampling are recommended to develop an understanding of temporal
variability141 to ensure that final risk management decisions are based upon a consideration of a
reasonable maximum vapor intrusion condition.142
6.4.1 Indoor Air Sampling
Indoor air sampling results are used to assess the presence of and level of human health risk
posed by vapor-forming chemicals in indoor air (see Sections 6.3.4 and 7.4), and to diagnose
whether vapor intrusion is occurring (see Sections 6.3.3,6.3.5, and 7.3). These two uses of
indoor air sampling in vapor intrusion investigations are discussed further belowwith
recommended methods for each. As discussed furtherin Sections 8.4 and 8.7, indoor air
sampling may also be useful for supporting performance evaluations of vapor intrusion
mitigation systems and verifying the health protectiveness of subsurface remediation systems.
EPA recommends that the decision to collect indoor air data be supported by lines of site- or
building-specific evidence (e.g., characterization of subsurface vapor source(s) strength and
proximity to building(s), vadose zone conditions, and building conditions) that demonstrate that
vapor intrusion has the potential to pose a significant human exposure. Confidence in the
assessment is expected to be higher when multiple lines of site- or building-specific evidence, in
addition to indoor air data, come together to provide mutually supporting evidence for a common
understanding of the site conditions/scenarios and the potential for vapor intrusion (i.e., the
various lines of evidence are in agreementwith each other).
A potential shortcoming of indoor air testing is that indoor sources and outdoorsources
unrelated to subsurface contamination and to releases from the subject site - "background" (see
Glossary) - may contribute to the presence of volatile chemicals in occupied buildings (see
Section 2.7), particularly if these sources cannot be removed from the building prior to and
throughout the duration of sampling indoors. This shortcoming of indoor air testing is
Seasonally variable conditions (e.g., moisture levels, depth to groundwater) can lead to seasonally variable
concentrations and distributions of vapors n the vadose zone. Likewise, weather conditions and building operations
can lead to time-variable contributions from vapor intrusion (e.g., driving forces for vapor intrusion; see Section 2.3)
and ambient air infiltration (see Sections 2.4). Collectively, these processes cause indoor air concentrations of vapor-
forming chemicals to vary over time (see Section 2.6). An individual sample (or single round of sampling) would be
insufficient to characterize seasonal variability, or variability at any other time scale.
142 EPA recommends basing the decision about whetherto undertake response action forvapor intrusion (i.e., a
component of risk management; see Section 7.4) on a consideration of a reasonable maximum exposure (e.g., EPA
1989, 1991 a), which is intended to be a semi-quantitative phrase, referring to the lower portion of the high end of the
exposure distribution (see Glossary).
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unavoidable when the subsurface environment contains the very same volatile chemicals that
contemporaneously arise in indoorair due to background sources, which is common for some
chemicals and relatively rare for others (EPA 2011 a). In this circumstance, additional lines of
evidence, possibly including special procedures and analyses, may warrant evaluation to
distinguish background contributions from those originating from vapor intrusion (see Section
6.3.5).
After discussing recommended sampling methods and practices for the primary uses of indoor
air sampling data, this sub-section concludes by discussing:
Recommended measures to reduce the impact of indoor sources of vapor-forming
chemicals.
Recommended approach to establishing analyte lists for indoor air samples.
Complementary, building-specific data (i.e., additional lines of evidence) that can be
collected contemporaneously while indoors.
Characterize Human Exposure Levels. Indoor air sampling and analysis provide a direct
approach to obtaining concentrations of vapor-forming chemicals in indoor air to which building
occupants can be exposed. For this purpose, EPA generally recommends time-integrated
sampling methods, since indoor air concentrations can be temporally variable143 and time-
integrated exposure estimates over appropriate exposure durations (e.g., chronic typically; less-
than-chronic in some cases) are generally most useful for assessing human exposure and
human health risk (see Section 7.4).
Because of variability, a single indoor air sample, collected at a randomly chosen time, is
insufficient information to estimate an average exposure. On the other hand, it is impractical to
collect indoor air samples continuously over a chronic exposure period (i.e., up to 30 years fora
reasonable maximum exposure duration in a residence (EPA2014a)), which would also entail
deferring risk management decisions for a prolonged period while human exposures from vapor
intrusion could occur unabated. Hence, current and past practice has generally relied upon
collecting multiple indoor air samples for purposes of estimating long-term average (i.e.,
chronic) exposures and assessing human health risk (see Section 7.4). All else being equal, a
longer collection period for each individual sample would be expected to yield a more reliable
basis for estimating long-term, time-average exposure than would a one-day sample collection
period.
When investigating short-term exposure conditions that might warrant prompt response action
to protect human health (see, for example, Sections 5.2 and 7.5.2), time-integrated indoor air
samples can provide useful estimates of exposure for the location and time period of sample
collection. (A short-term exposure is defined as a "repeated exposure for more than 24 hours,
Because weather conditions and building operations can lead to time-variable contributions from vapor intrusion
(e.g., driving forces for vapor intrusion; see Section 2.3) and ambient air infiltration (see Sections 2.4), indoor air
concentrations of vapor-forming chemicals can be expected to vary over time (see Section 2.6). Field observations
indicate that indoor air concentrations arising from vapor intrusion can be temporally variable within a day and
between days and seasons in an individual residential building (EPA 2012f; Holton etal., 2013ab).
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up to 30 days" - see Glossary.) All else being equal, a longercollection period for an individual
sample would be expected to yield a more reliable basis for estimating time-average short-term
exposure than a one-day sample.
As noted above, EPA also recommends considering potential health effects and relevant
exposure periods for site-related, vapor-forming chemicals when developing DQOs and
sampling plans for indoor air.
Variability in laboratory analyses can be considered when evaluating these data in support of
risk management decisions.144
Time-integrated samples provide a direct measurement of the average chemical concentration
over a fixed period of time (e.g., ranging from 8 hours to several weeks, depending upon the
sampling method, its capabilities, and its deployment). Time-integrated samples can be
collected using either evacuated canisters, which collect gas in a container, or sorbent
samplers, which collect vapor-forming chemicals on a sorbent material.
Evacuated canisters
Evacuated canisters are spherical- or cylindrical-shaped stainless steel or silica-lined
containers that are prepared to be under negative pressure relative to the environment and
certified by the laboratory to be clean and leak-free.145 As described in EPA Method TO-14A
(EPA 1999c), evacuated canisters can be used as passive (sub-atmosphericpressure
sampling) or active (pressurized sampling) samplers. In both cases, the canister is initially
evacuated to a standard vacuum in preparation for sampling. Forsub-atmospheric
sampling, when the canister is opened for sample collection, the differential pressure causes
air to flow into the canister without use of a pump. In this case, sampling must end before
the vacuum is fully dissipated, else the sample collection period will be unknown. For
pressurized sampling, a pump is used to pass air into the canister until a specified pressure
(up to two atmospheres) is reached. In both cases, a flow-control device is used to maintain
a constant flow into the canister over the desired sample period. To ensure that the
canisters are filling at the proper rate, EPA recommends checking the flow rate periodically
during sample collection. EPA Methods TO-14A and TO-15 provide further information on
measuring and controlling flow rates into canister-type samplers.
144 For a recently published study, EPA's ORD determined "The acceptance criterion to demonstrate equivalency is
+30% ... based on what is defined as acceptable reproducibility in vapor intrusion field studies" (EPA 2012f).
145 Canisters are cleaned and re-used because they are too expensive to dispose routinely. The certification process
entails cleaning the interior of the container using a combination of dilution, heat, and high vacuum. Canisters are
then analyzed fora large suite of vapor-forming chemicals to establish that they are free of detectable chemicals at a
suitably low (sensitive) detection level. The cleanliness of canisters can be certified individually (i.e., direct testing of
each canister, or 100% certification), which is generally desirable in instances where the data are to be used for
exposure/risk assessment purposes. Alternatively, canisters can be certified by batch (or lot), in w hich a subset of the
canisters are tested directly (e.g., 10%) and results are extrapolated to the remainder of the batch. Batch-certified
canisters may be sufficient when concentrations of target analytes are expected to be high, relative to potential levels
of residual contamination in the canister after cleaning. EPA recommends that flow controllers also be cleaned
between uses to avoid artificial contamination.
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Typically, for vapor intrusion investigations, indoorair samples are collected using six-liter
canisters using sub-atmospheric pressure sampling over a 24-hour period in residences or
over an 8-hour period (or workday equivalent) in commercial and industrial settings, when
using these devices. Larger canisters (i.e., 15-liter) allow higher flow rates and may be
preferable for longer sampling events or to collect a larger volume of sample.146 A capillary
flow controller has been developed and demonstrated for use in industrial hygiene
applications (Rossneret al. 2002; Rossnerand Wick 2005), which may hold promise for
extending sampling time periods for indoor air with standard-sized canisters.
Details for selecting and utilizing sampling canisters are provided in EPA Methods TO-14A
(EPA 1999c) and TO-15 (EPA 1999d). EPAs Environmental Response Team has
developed a standard operating procedure for sampling air with evacuated canisters (EPA-
ERT1995).
An advantage of using evacuated canisters for sample collection is the capability of
analyzing multiple sub-samples from the same canister (because these canisters obtain a
"whole air" sample). They are also reasonably easy to deploy and retrieve. However, sample
recovery and representativeness can be affected by ambient conditions; for example, low
humidity conditions in the sample may lead to losses of certain volatile compounds on the
canister walls (EPA 1999c).
Fourteen days is the most commonly cited hold time for air samples in canisters. Some
analytes, however, may be stable in canisters for up to 30 days.
Sorbent Samplers
Sorbent sampling devices are hollow containers that hold one or more adsorbent media that
can bind vapor-forming chemicals. They have been developed and tested over several
decades for industrial hygiene monitoring and have more recently been employed for other
purposes, including vapor intrusion investigations. Sorbent samplers can be used in an
active or passive mode.
In the active mode, a pump is used to draw air at a known rate through the device. Theflow
rate and sampling volume are determined based on the type of sorbent used, the target
constituent(s), and the amount of sorbent contained in the device. Care must be taken to
ensure that the volume of air drawn through the tube does not exceed the "breakthrough"
volume147 (i.e., the volume of air which may be passed through the sorbent tube before a
detectable level of the analyte concentration elutes from the non-sampling end), else the
time-weighted average concentration will be biased low by an unknown amount.
In the passive mode, no pump is used, and vapor-forming chemicals enter the device due to
diffusion. Consequently, passive (diffusion) samplers may be placed in locations of interest
without consideration of power availability.
Alternatively, two (or more) large canisters can be connected together to allow collection of time-integrated
samples over longer durations, which is generally desirable for characterizing long-term average exposure levels.
For this reason, sample volumes in the one- to four-liter range are generally recommended for this method (EPA
1999e).
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Although passive (diffusion) samplers have been less commonly used to quantify indoorair
concentrations, their use may growas a result of recent demonstrations thatthey can yield
results comparable to those obtained using evacuated canisters (EPA-Region 9 2010; EPA
2012f; Odencrantzetal. 2009; Odencrantzetal. 2008),148 and a recognition that they may
be less intrusive for some building owners and occupants and more convenient for field staff
(EPA-Region 9 2010). Passive samplers are also capable of being deployed for longer
durations than evacuated canisters, thereby providing a more economic means of obtaining
average indoor air concentrations over longer periods of exposure. Time-integrated samples
of indoor air over longer periods than one day are also indicated by field observations
demonstrating that indoor air concentrations arising from vapor intrusion can be temporally
variable within a day and between days and seasons (EPA 2012f; Holton etal., 2013a).
The basic configuration of a passive sampler is a solid, typically granular, sorbent contained
in a metal, glass of plastic container with openings of known dimensions. Several different
containers and a wide range of adsorptive media are commercially available, which function
similarly. After sample collection, adsorbed mass is measured in a laboratory for each
analyte; the two most common analytical methods involve thermal desorption or solvent
extraction combined with gas chromatography/mass spectrometry. The air concentration for
each analyte is calculated from the adsorbed mass, the duration of sampling, and the
uptake rate.
The uptake rate is dependent upon the geometry of the sampling device and the diffusion
coefficient of the analyte. The uptake rate is the most critical variable for accurately
measuring air concentrations with passive samplers, since the sampling duration and
adsorbed mass can generally be measured very accurately. Fortunately, most commercially
available passive samplers have published uptake rates for several compounds, which
collectively address many of the vapor-forming chemicals described in Section 3.1.149 Once
the target analyte(s) and uptake rate(s) are known, the sample duration needed to attain
data quality objectives (i.e., reporting limit equal to or lower than the risk-based screening
level or risk-based action level) can be calculated for each analyte.
Uptake rates of deployed samplers can be affected by ambient conditions (e.g.,
temperature, because chemical-spedfic diffusion rates are temperature-dependent; and
humidity, which influences the uptake of water vapor, which may interfere with retention and
stability of the analyte and/or with laboratory analysis). EPA, therefore, recommends that
ambient conditions be recorded during deployment of passive samplers.
One potential advantage of passive samplers is that they can be left unattended for
relatively longer durations, thereby conveniently providing estimates of longer-term time-
For example, EPA's National Exposure Research Laboratory found that Radiello charcoal passive samplers
performed well for sampling periods up to 28 days for TCE (EPA 2012f). In that study, one-week (7-day) Radiello
passive samplers were utilized as a primary measurement tool and the resulting data were used as a basis of
comparison to longer-duration samples (e.g., two-week,four-week(monthly), and 13-week(quarterly) samples).
149
Standard methods for determining uptake rates have been published by a few organizations. Ideally, the selected
passive sampling device will have vendor-supplied uptake rates supported by controlled chamber tests or a
considerable body of field-calibrated uptake rates for most, if not all, of the target compounds.
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weighted average concentrations. However, similar to active sorbent sampling, the duration
of passive sampling must be such that the adsorptive capacity of the media is not
approached or exceeded, else the time-weighted average concentration will be biased low
by an unknown amount.
One potential disadvantage of sorbent sampling, compared to canistersampling, is that only
one analysis is possible from an individual device, because it does not collect a "whole air"
sample. Thus, if an error occurs in laboratory handling or there is an instrument malfunction,
the sample is lost. Such errors generally are not common. Therefore, this potential
disadvantage will not generally offset the benefits of sorbent sampling, the foremost of which
is the ability to obtain time-integrated samples over longer periods (i.e., up to a few months
for some compounds) than with evacuated canisters.
For a typical-size residential building or a commercial building less than 1,500 square feet, EPA
recommends that the site teams generally collect one time-integrated sample in the area directly
above the foundation floor (basement or crawl space) and one from the first floor living or
occupied area, at least for the initial sampling round. 15ฐ In general, EPA recommends samples
be collected at the breathing zone level for the most sensitive exposed population.151
EPA recommends the site planning and data evaluation team discuss the number of sample
locations per building for atypical situations, which include: (1) very large homes or buildings;152
(2) multi-use buildings, particularly ones with segmented areas that are occupied by different
populations (e.g., day care with young children versus office with adult workers) or have
different occupancy patterns overtime. Additional samples may also be warranted, depending
on internal building partitions, HVAC layout, contaminant distribution in the subsurface, and
occurrence of observable locations of potential soil gas entry (e.g., basement sumps or drains,
relatively large holes or spaces in the foundation floor, entry points for utilities). Closed rooms
located belowground may have appreciably highercontaminant concentrations originating from
vapor intrusion. Closed rooms may warrant sampling to characterize the reasonably maximum
exposure levels, if occupied, or to diagnose vapor intrusion (e.g., see below), even if not
occupied.
150
Placement of indoor air sampling devices may entail compromises. Whereas the ideal location may be a central
location that is unobstructed and representative of the actual used area of a room, placement at breathing zone
height in a heavily used area well away from any wall is likely to interfere w ith normal occupant activities.
151
The "most sensitive" exposed population may be identifiable by combining information about the types of human
occupants in a given building and the types of potential toxic effects for vapor-forming chemicals found in the
subsurface environment. For example, the 'most sensitive' exposed population could be children, pregnant women,
or elderly adults, depending upon building- and chemical-specific characteristics.
152
Larger commercial and residential buildings (e.g., multi-family residences) may warrant additional discussion with
the site planning team and perhaps a statistician to select the appropriate number and placement of indoor air
samples to meet DQOs.
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Indoor air concentrations vary over time, due to time-dependent changes in soil gas entry rates,
exchange rates, intra-building mixing, among otherfactors (see Section 2).153 Therefore,
multiple rounds (and often several rounds) of indoor air sampling is generally recommended in
order to reduce the chance of reaching a false-negative conclusion (i.e., concluding exposure is
at an acceptable risk level when it is not) or reaching a false-positive conclusion (i.e., concluding
exposure is at an unacceptable risk level when it is not).154 Also, multiple sampling events
generally are considered necessary to accountfor seasonal variations in climate and the habits
of building occupants and ensure that related risk management decisions are based upon a
consideration of a reasonable maximum vapor intrusion condition.155 In many geographic areas
in the continental United States, indoor air sampling during the heating season may yield higher
indoor air concentrations than at other periods, because stack effects are generally more
significant and, therefore, higherrates of soil gas entry are reasonably expected. Another
scenario that may yield higher indoor air concentrations is when a building is sealed and the
ventilation system is not operating.
When sampling indoor air (or sub-slab soil gas), EPA generally recommends removing potential
indoor sources of vapor-forming chemicals (see Section 2.7) from the building to strive to
ensure that the concentrations measured in the indoorair samples are attributable to the vapor
intrusion pathway.156 After removal of indoorsources, their effects may linger longerdepending
on source strength, relative humidity inside the building, the extent to which the contaminants
have been absorbed by carpets and other fabrics or "sinks," and air exchange rate of the
building. In residential settings, EPA generally recommends that potential indoor sources be
removed from the structure and stored in a secure location at least 24 to 72 hours priorto the
start of sampling, based on an approximate air exchange rate of 0.25 to 1.0 per hour in
-|CO
Because weather conditions and building operations can lead to time-variable contributions from vapor intrusion
(e.g., driving forces for vapor intrusion; see Section 2.3) and ambient air infiltration (see Sections 2.4), indoor air
concentrations of vapor-forming chemicals can be expected to vary over time. Holton et al. (2013ab) obtained and
reported a set of long-term, high-frequency, indoor air data for an unoccupied house (except for periodic visits by
researchers) in Utah overlying a plume of chlorinated hydrocarbons. TCE concentrations in indoor air varied by
approximately twoto three orders of magnitude, exceeding variations in measured air exchange rate.
154
An individual sample, collected at a randomly chosen time, may under-estimate average and reasonable
maximum exposure conditions. From their high-frequency, measured data, Holton et al. formulated a synthetic data
set (simulating one-day-average concentrations), which they used to estimate that a single, randomly drawn, one-day
sample had a forty percent chance of being less than the true mean (Holton et al. 2013b; see Table 1 therein). When
the true mean was assumed to exceed the risk-based action level ("target concentration" in their parlance) by two or
five times, they estimated that a single, randomly drawn, one-day sample had a twenty percent or six percent chance,
respectively, of not detecting the exceedance. These data support EPA's recommendation to collect multiple rounds
of indoor air sampling to reduce the chance of reaching a false-negative conclusion. Collecting multiple rounds of
indoor air sampling can also reduce the chance of reaching a false-positive conclusion (i.e., concluding that vapor
intrusion poses unacceptable human health risk when it does not), because an individual sample, collected at a
randomly chosen time, may over-estimate the average exposure condition.
155 Given EPA's over-arching duty to protect human health and recognizing the disruption to building owners and
occupants caused by indoor air sampling, risk managers may choose to pursue pre-emptive mitigation (i.e., early
action) at some buildings (see Sections 3.3 and 7.8) rather than, for example, conduct multiple rounds of sampling
over a few years to establish an estimate of long-term average exposure concentration and characterize temporal
variability.
156
Vapor-detecting field instruments and in-field gas chromatographs can be used to locate indoor sources of vapors.
For example, Gorder and Dettenmaier (2011) reported on the use of a field-portable gas chromatograph and mass
spectrometer to identify specific sources of vapor-forming chemicals. EPA's Environmental Response Team has
employed the Trace Atmospheric Gas Analyzer (TAGA) mobile laboratory for similar purposes.
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residential buildings. In residences with attached garages, keeping the door(s) between the
garage and the living space closed prior to and during indoorair sampling may also be
warranted, in situations where the site-spedfic chemicals of potential concern include petroleum
hydrocarbons or are components of products stored in the garage.
Diagnose Vapor Intrusion and Background Sources. When access is granted for indoor air
sampling, EPA generally recommends concurrently collecting samples of sub-slab soil gas (see
Section 6.4.3) and outdoor (ambient) air (see Section 6.4.4) over similar durations using the
same methods. Comparing these results to each other and to results for subsurface vapor
sources can foster insights and support findings about the relative contribution of vapor intrusion
and background sources to indoor air concentrations (as described in Section 6.3.5). In this
case, time-integrated sampling methods are recommended for indoor air, because
concentrations of vapor-forming chemicals can vary significantly overtime (see Section 2.6).
Grab (essentially short-duration) samples can, however, be useful for:
confirming the presence of a subsurface contaminant in indoor air157 (see Section 6.3.4)
or in gas in a drain line or sewer lateral that enters a building,
identifying specific vapor-forming chemicals emanating from indoor sources of consumer
or commercial products158 (see Section 6.3.5), and
identifying specific vapor-forming chemicals emanating from suspected openings for soil
gas entry into buildings (see Section 6.3.3).
Grab samples can provide a convenient and less intrusive means of confirming the presence, if
any, of a site-related subsurface contaminants) in the indoor environment. However, an
individual grab sample is not reliable for purposes of demonstrating that vapor intrusion is not
occurring in a specific building; among otherconsiderations, vapor intrusion and indoorair
concentrations can exhibit significant temporal variability (EPA 2012f, Holtonetal., 2013ab).
Consequently, EPA recommends collecting multiple time-integrated samples to support any
such building-specific determination.
Indoor air samples can also be concurrently collected for radon testing, which may be useful in
evaluating building susceptibility to soil gas entry (see Section 6.3.3).
Evaluate and Develop Analvte Lists. EPA recommends the site planning and data evaluation
team generally limit chemical analyses to those vapor-forming chemicals known (based upon
subsurface contaminant characterization) or reasonably expected (based upon site history) to
be present in the subsurface environment. Forexample, if the site history and reliable
subsurface sampling data do not identify benzene as a subsurface contaminant, it would be
appropriate forsite managers to exclude benzene as a targetanalyte for indoor air samples.
Benzene could originate indoors as a result of a car, lawnmower, or snow blower in a garage. In
For this purpose, EPA generally recommends collecting one sample directly above the foundation floor (e.g.,
basement or craw I space) and one from the firstfloor living or occupied area.
158 For ch;
sufficient.
1^8
For characterizing indoor sources or openings for soil gas entry, one round of grab sampling of indoor air may be
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this hypothetical case, benzene would not typically be amenable to reduction by vapor
mitigation systems or subsurface remediation efforts. In fact, requesting an extensive list of
analytes that are not related to subsurface contamination may unnecessarily complicate risk
communication if indoor air testing reveals volatile chemicals unrelated to vapor intrusion.
Collect Complementary Data While Indoors. A variety of useful information can be gathered
during a building survey conducted in advance of or during indoorair sampling. EPA
recommends that the following complementary data be gathered by observation, interviews, or
reports (e.g., mechanical test-and-balance reports) when buildings are to be sampled to analyze
indoorair:
Building Occupancy
o Characteristics and locations of building occupants (e.g., residents, including
children or other sensitive populations; expectations for presence of general
public in commercial or industrial settings; presence of multiple exposure units -
due to different uses or activities and occupants- within a building other than a
single-family residence).
o Hours of building occupancy under current conditions (and reasonably expected
future conditions, as appropriate), particularly for a nonresidential setting.
Because this information is pertinent to the human health risk assessment and
data evaluation, EPA recommends considering hours of building occupancy
when establishing the sampling duration for characterizing indoor air exposure
levels.
Susceptibility to Soil Gas Entry Under Current Conditions
o EPA recommends that the pressure difference between the indoors and the
subsurface be measured whenever indoor air samples are collected. Ideally,
differential pressure data would be collected continuously starting several days
before sampling and throughout the sample collection period.159 The magnitude
and direction of the pressure difference during sampling can support insights
about whether a 'driving force' for vapor intrusion is present during indoor air
sampling; if not, then the resulting sampling data are unlikely to characterize a
reasonable maximum vapor intrusion exposure condition. Differences in driving
forces (direction or magnitude) among indoorair sampling events may help to
explain any significant differences in observed indoor air concentrations over
time.160 Measuring pressure difference between the indoors and the subsurface
159
These data can be collected using portable pressure monitors installed in a dedicated sub-slab probe at one or
more locations. Pressure transducers were employed forthis purpose during high-frequency sampling as part of a
research study in Indianapolis (EPA 2012f, see Section 3.6.6 therein); readings were recorded every 15 minutes.
Technical information about pressure-measuring instruments (e.g., description, operation, and calibration) can be
found in Section 4 of Technical Guidance Document: Compliance Assurance Monitoring (EPA 1998).
160
Pre-mitigation measurements of the pressure difference between indoors and the subsurface may also be useful
forsupporting design of active depressurization technologies to reduce vapor intrusion (EPA 1993a, see Section 3
therein).
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is a more direct means of assessing building under-pressurization than is
monitoring weather/climate factors (e.g., air temperature, wind speed). Pressure
difference monitoring in large buildings can help identify any areas with
significant under-pressurization.
o Presence and operation of a mitigation system, which would generally be
expected to mitigate intrusion of vapor-forming chemicals even if designed for
radon.
o Physical conditions that indicate potential openings to soil gas entry (e.g.,
potential conduits, such as cracks or floor drains; presence of structures such as
utility pits, sumps, and elevators; basements or crawl spaces; modifications to
the original foundation).
Building Heating, Ventilation, and Cooling
o Building ventilation, including zones of mechanical influence and stagnation. As
noted in Section 2.4, greater ventilation is intended to result in smaller vapor
concentrations in indoor air. Any non-ventilated or passively ventilated rooms
(such as mechanical rooms) may be subject to greater accumulation of vapors.
For commercial and industrial buildings, each distinct zone of influence may
warrant sampling, when indoor air testing is selected as part of a site-specific
investigation plan for vapor intrusion assessment.
o Operating characteristics of HVAC systems. In many commercial buildings, the
HVAC system brings outdoor air into the building, potentially creating building
over-pressurization relative to the outdoor environment. EPA recommends noting
any areas with significant over-pressurization, relative to the outdoors.
Indoor and Outdoor Sources of Vapor-Forming Chemicals
o Chemicals and consumer products used or stored within the building that can act
as potential sources of toxic vapors. Vapor-forming chemicals are used in many
commercial and most industrial buildings.161 As noted in Section 2.7, consumer
products that can emit vapors may be common in residential buildings. In some
circumstances, a photoionization detector (PID) can be used to directly screen
the building for locations with vapor-forming chemicals and materials; however,
the PID may not be sensitive enough for very low concentration sources. More
sensitive options include use of the HAPSITE gas chromatograph/mass
spectrometer (Gorder and Dettenmaier 2011) or the TAGA Mobile Laboratory
(EPA-ERT2012).
o HVAC systems that bring outdoor air into the building potentially bring
contaminated outdoor air into the building, depending on the location of the vent
and exhaust with regard to other spaces. For example, HVAC intakes adjacent to
Depending upon it history of uses and operations, buildings undergoing renovation, redevelopment or reuse may
have lingering presence of vapor-forming chemicals due to a past release(s) also.
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or near a dry-cleaning facility may introduce vapors of the dry-cleaning solvent
into the building.
In some cases, contaminated groundwater seeps into or actively collects in the building
(for example, in sumps), possibly serving as a direct source of vapors. It may be
appropriate to collect water samples concurrently with indoor air (and any sub-slab)
samples in these circumstances.
Presence and operation of any indoorairtreatmentsystem (e.g., in-linecarbon
adsorption) that can reduce indoorexposure levels of vapor-forming chemicals.
In general, EPA recommends that the foregoing complementary information be collected during
investigation planning and scoping to help determine where to sample and prioritize or
sequence buildings for testing. Then, the information can be confirmed during indoor sampling.
Field experience in residential settings suggests that it may not be possible to remove all indoor
sources of vapor-forming chemicals. It may be particularly impractical to do so in industrial
settings where vapor-forming materials are used or stored. It may also be impractical when
deploying passive samplers, owing to their longerdeployment period. Therefore, EPA
recommends asking building occupants to document indoorsources (and relevant building
operations) during indoor air sampling, using an activity log or questionnaire.
6.4.2 Outdoor Air Sampling
Outdoor air concentration data can be useful in identifying potential contributions to indoor air
concentrations from ambient air sources (see Section 6.3.5). Therefore, EPA generally
recommends collecting ambient air samples using similar sampling and analysis methods,
whenever indoor air samples are collected. Normally, EPA recommends one or two outdoor air
sample locations to characterize the conditions surrounding a single or a few buildings.162
Additional outdoor air samples may be warranted if the investigation is assessing multiple
buildings over a wide area. EPA also recommends that sample locations be designed to
characterize representative conditions in the absence of site-related subsurface contamination
(e.g., avoid collecting ambient air samples near locations of known or suspected chemical
release(s), including any atmospheric releases from remediation equipment). It also is
suggested that observable potential outdoor sources of pollutants (e.g., air emissions from
nearby commercial or industrial facilities) be recorded during all building surveys.
Because concentrations of vapor-forming chemicals in ambient air can vary with time, EPA
recommends that ambient air samples generally be collected overthe same sampling period as
indoor air, which will facilitate data evaluations when contaminant concentrations are compared
between media. For residential buildings, EPA generally recommends beginning ambient air
sampling at least one hour, but preferably two hours, before indoor air monitoring begins and
continuing to sample until at least 30 minutes before indoormonitoring is complete. EPA
recommends this practice because most residential buildings have an hourly air exchange rate
in the range of 0.25 to 1.0, causing air that enters the building before indoor air sampling to
For buildings where outdoor air is mechanically brought into the building, an outdoor sample may be co-located
near the HVAC intake.
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remain in the building for a long time (for example, see Section D. 10, ITRC 2007a).
Recommended lag times may warrant adjusting for nonresidential buildings.
Evaluate and Develop Analvte Lists. To characterize potential concentrations entering a building
via ambient air, EPA generally recommends that chemical analysesfor ambient air samples be
limited to those vapor-forming chemicals known (based upon subsurface testing) or suspected
(based upon site history) to be present in the subsurface environment. Requesting an extensive
list of analytes that are not related to subsurface contamination, as discussed previously, may
unnecessarily complicate risk communication.
Consider Collecting Complementary Data. Monitoring air exchange during ambient air sampling
events can provide useful complementary data. Ideally, these data would be collected
continuously starting before sampling and throughoutthe sample collection period. Information
about air exchange can support insights about the amount of ambient air infiltration during
sampling.
6.4.3 Sub-slab Soil Gas Sampling
Sub-slab sampling is intended to draw soil gas from the airspace immediately belowthe floor
slab of a building. Depending upon building construction and condition, this air space may be an
air gap that forms beneath a concrete foundation due to differential settlement over time or a
pore space within a granular layer that may have been placed belowthe concrete slab. Access
to this air space is generally provided by drilling or coring through the concrete and inserting a
probe, which is sealed into the floor. EPAs Environmental Response Team has developed a
standard operating procedure for constructing and installing sub-slab soil gas sampling probes
(EPA-ERT2007).
Sub-slab soil gas samples can provide useful data for characterizing the levels of hazardous,
vapor-forming chemicals that can enter a building via soil gas intrusion. When combined with
other soil gas data, sub-slab soil gas data can be used to assess whetherthe subsurface vapor
migration route is complete (i.e., subsurface vapor migration is capable of transporting
hazardous vapors from the source to building; see Section 6.3.2). When combined with an
appropriate attenuation factor (e.g., a conservative generic value- see Section 6.5.3), sub-slab
soil gas data can be used to estimate a potential upper-bound indoorair concentration163 that
may arise from vapor intrusion. In this way, sub-slab data can be used to assess the potential
for the vapor intrusion pathway to pose a health concern.164
For purposes of this Technical Guide, the term "upper bound indoor air concentration" is intended to be a semi-
quantitative phrase, referring to the high end of the exposure distribution. EPA recommends basing the decision
about whetherto undertake response action for vapor intrusion (i.e., a component of risk management) on a
consideration of a "reasonable maximum exposure" (e.g., EPA 1989, 1991 a), which is intended to be a semi-
quantitative phrase, referring to the lower portion of the high end of the exposure distribution (see Glossary).
Alternatively, a "worst case" or "reasonable worst case" (see Glossary) indoor air concentration would refer to the
upper portion of the exposure distribution. Section 6.6, which discusses mathematical modeling of vapor intrusion,
notes that consideration of a "worst case" exposure condition may be particularly useful where the predicted "worst
case" indoor air concentrations can be shown to pose acceptable human health risk.
164
The sub-slab soil gas concentration provides only half of the information for estimating vapor flux into a building.
The other information needed is the soil gas flow rate (Qsoii), which is embodied in the attenuation factor. The soil gas
flow rate can also be explicitly calculated using a model.
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Field experience indicates there may be substantial spatial variability in sub-slab soil gas
concentrations even over an average-sized footprint of a residential building. EPA, therefore,
recommends site planning and data reviewteams consider collecting multiple samples per
building when sub-slab soil gas sampling is conducted.165 Three sub-slab samples have been
collected in a number of EPA investigations of a typical size residential building or commercial
building less than 1,500 square feet in area. EPA recommends the site planning and data
evaluation team discuss the number of sample locations perbuilding for atypical situations,
which include: (1) very large or small homes or buildings;166 (2) buildings with more than one
foundation floor type; (3) subsurface structures or conditions that might facilitate or mitigate
vapor intrusion; and 4) multi-use buildings with distinct segmented areas that differ significantly
by occupying population or exposure frequency. In addition, EPA recommends multi-point sub-
slab samples be considered to support data interpretation and resolve uncertainties that may
arise when:
There are fewer surrounding buildings that are being sampled (that could have helped
the understanding of typical sub-slab values and variability).168
The indoor and sub-slab concentrations fora specific building(s) are out of line with
expectations based on data from neighboring homes and other information.
EPA generally recommends that sub-slab sampling include centrally located sub-slab samples
in buildings identified for testing when the subsurface vapor source is laterally extensive relative
to the building footprint (e.g., a broad plume of contaminated groundwater).169 In addition, EPA
recommends that site teams consider internal building partitions, HVAC layout, contaminant
distribution, utility conduits, and openings for preferential soil gas entry in selecting any
additional locations for collecting sub-slab samples.
Several rounds of sampling are generally recommended to develop an understanding of
temporal variability of sub-slab soil gas concentrations, particularly when these data are used
with the recommended attenuation factor (see Section 6.5.3) to estimate a potential upper-
bound indoor air concentration that may arise from vapor intrusion.
An individual sample, collected at a randomly chosen time, may under-estimate or over-estimate average subslab
conditions. Collecting multiple subslab soil gas samples can, therefore, reduce the chance of reaching a false-
negative conclusion (i.e., concluding subsurface vapor source strength is limited, when vapor intrusion actually poses
an unacceptable human health risk) or a false-positive conclusion (i.e., concluding subsurfacevapor source strength
is unacceptably elevated, when vapor intrusion actually poses an acceptable human health risk).
166
For larger structures, a statistician may assist in identifying the number and placement of sampling ports to meet
the desired DQOs.
167 In basements with a partial slab, but one large enough to allow vapors to accumulate (for example, if the slab
covers more than 50 percent of the building footprint), EPA generally recommends that one sub-slab port be installed
on the slab portion and an indoor air sample be collected directly over the dirt portion.
In these cases, EPA recommends multiple ports be installed in a specific percentage (e.g., more than 10 percent)
of the buildings sampled to provide a check for variability in the study area.
Based on workconducted in New York as of the spring of 2010, it appears that the sub-slab concentrations
beneath the central area of a home are usually (75 percent of the time) higher than (or as high as) the concentrations
closer to the perimeter of the home. This field observation is supported by modeling results for idealized scenarios,
which show greater sub-slab soil gas concentrations near foundation centers in under-pressurized residential
buildings when the vapor source is laterally extensive relative to the building footprint (EPA 2012b).
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If a site team decides to proceed with sub-slab sampling, EPA recommends that leak-testing be
performed to ensure the hole is properly sealed, for example through the use of a helium tracer
gas shroud. Because installing soil gas probes can disturb subsurface conditions, EPA
recommends that the site team allow some time after the sampling probe has been installed for
the subsurface to return to equilibrium conditions. An EPA study of the time needed for the
subsurface conditions to come back to equilibrium (equilibration rate) afterthey have been
disturbed by installation of the soil gas probes found that an equilibration time of two hours
generally was sufficient because most sub-slab material consists of sand or a sand-gravel
mixtureeven for buildings built directly on clay (Section 5.0, EPA 2006b).
There also may be special considerations for sub-slab soil gas samples because of either a
unique construction (for example, pretension concrete slab) or environmental situation. Key
EPA recommendations include, but are not limited to:
Identify the location of cables in post-tensioned concrete (e.g., using ground-penetrating
radar) before sub-slab sampling, as drilling through a cable poses a significant health
and safety concern and may damage the slab.
Avoid locating sub-slab samples in areas where groundwater might intersect the slab.
Identify and avoid the location(s) of underground utilities and structures (for example,
electric, gas, water, or sewer lines) to prevent damage to these lines; however, sample
collection in close proximity to these lines may be warranted as building penetrations for
these lines may pose openings for soil gas entry.
Consider whether to augment sub-slab samples with samples through the basement
walls, as the primary entry points for vapors in basements might be through the
sidewalls rather than from belowthe floor slab.
Evaluate and Develop Analvte Lists. To characterize potential concentrations entering a building
via soil gas, EPA generally recommends that chemical analyses for sub-slab soil gas samples
be limited to those vapor-forming chemicals known (based upon subsurface testing) or
suspected (based upon site history) to be present in the subsurface environment. Requesting
an extensive list of analytes that are not related to subsurface contamination, as discussed
previously, may unnecessarily complicate risk communication.
Collect Complementary Data While Indoors. When sub-slab soil gas samples are collected,
EPA recommends that the following complementary information be gathered by observation or
interviews:
Physical conditions and characteristics that are pertinent to assessing the building's
susceptibility to soil gas entry, if any (e.g., potential conduits, such as cracks orfloor
drains; presence of structures, such as utility pits and elevators; basements or crawl
spaces). Such information may help interpret spatial differences in sub-slab or indoor air
concentrationswithin a building.
Areas with potentially significant over- or under-pressurization relative to the outdoors.
Such information may assist in interpreting spatial differences in sub-slab or indoorair
concentrationswithin a building.
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Where outdoor air is mechanically brought into the building by the HVAC system and
building(s) interiors are over-pressurized, it may be helpful to also collect ambient air
samples to support interpretations of the sub-slab sampling results. If the predominant
vapor-forming substances and their respective concentrations in sub-slab soil gas and
outdoor air samples are similar, then ambient air may be influencing sub-slab soil gas
conditions.
EPA recommends that the pressure difference between the indoors and the subsurface be
measured whenever sub-slab soil gas samples are collected. Ideally, differential pressure data
would be collected continuously starting several days before sampling and throughout the
sample collection period. 17ฐ EPA recommends measuring pressure at locations away from
where sub-slab sampling probes are installed to avoid any pressure artifacts caused during
purging and sampling. The magnitude and direction of the pressure difference during sampling
can support insights about whether a driving force for vapor intrusion is present during
sampling.
When any sub-slab soil gas sample is collected, EPA recommends that relevant meteorological
data that can influence soil gas concentration patterns at the time of sampling, such as wind
speed, snowor ice cover, significant recent predpitation, and changes in barometric pressure,
be recorded, using direct observation (e.g., for snowor ice cover) or readily available data
sources (e.g., regional weather stations). These data may be helpful qualitatively in data
interpretation; for example, in reconciling soil gas data collected on multiple occasions.
A potential shortcoming of sub-slab soil gas testing is that gaining access may be difficult (or, in
some cases, infeasible). This difficulty can often be overcome by implementing a program of
community outreach and engagement that fosters trust and good relationships (see Section
9.0).
When access is granted for indoor sampling, EPA recommends collecting sub-slab and indoor
air samples contemporaneously using similar sampling and analysis methods and sampling
durations to allowfor data comparison. The sub-slab sampling ports can be installed afterthe
indoor air sample is deployed and collected (8 - 24 hours later) to avoid biasing the indoor air
concentrations with potentially higher sub-slab gas infiltration rates during port installation.
Alternatively, the sub-slab ports may be installed priorto indoor air sampling and sampled
concurrently with the indoor air samples, provided sufficient time is allowed for the indoor air
concentrations to return to "normal" after installation of the sub-slab port.171
170 These data can be collected using portable pressure monitors installed in a dedicated sub-slab probe at one or
more locations. Pressure transducers were employed forthis purpose during high-frequency sampling as part of a
research study in Indianapolis (EPA 2012f, see Section 3.6.6 therein); readings were recorded every 15 minutes.
Technical information about pressure-measuring instruments (e.g., description, operation, and calibration) can be
found in Section 4 of Technical Guidance Document: Compliance Assurance Monitoring (EPA 1998).
171 EPA generally recommends delaying indoor air testing for at least 24 to 72 hours based on an approximate air
exchange rate of 0.25 to 1.0 per hour. Note that the effects of any 'spike' in indoor air concentration may linger
depending on source strength, relative humidity inside the building, and the extent to which the contaminants have
been absorbed by carpets and other fabrics or "sinks."
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6.4.4 Soil Gas Sampling
Data obtained from a soil gas survey can be used to identify, locate, and characterize
subsurface vapor sources (see Section 6.3.1) and characterize subsurface vapor migration
routes, including any impedances from geologic, hydrologic, or biochemical conditions (see
Section 6.3.2). Soil gas survey data can also be useful in supporting the design of soil vapor
extraction systems and other subsurface remediation systems and the performance assessment
of these systems (see Section 8.1). For each of these purposes, EPA recommends that soil gas
survey data be supported by site-specific geologic information (i.e., site geology and subsurface
lithology).
Soil gas sampling generally consists of installing a probe into the ground, drawing gas out of the
probe, and collecting the gas for transport to a location for analysis. Inert materials (e.g.,
stainless steel, copper, brass, polyvinyl chloride, high-density polyethylene) are recommended
for constructing soil gas probes. To ensure that data collected are representative of conditions
in situ (e.g., are not adversely impacted by artificial infiltration of ambient air), a reliable seal of
the annulus between the probe and the probe housing and leak testing for the seal are generally
recommended. In addition, purging of the probe before collecting the soil gas sample is
recommended, analogous to purging of monitoring wells before collecting groundwater samples.
EPA's Environmental Response Team has developed a standard operating procedure for soil
gas sampling, including constructing and installing sampling probes (EPA-ERT 2001 c).
Typically, grab (rather than time-integrated) samples are collected when sampling soil gas. EPA
recommends that the site team allow some time after the sampler has been installed for the
subsurface to return to equilibrium conditions because installing temporary or permanent soil
gas probes can disturb subsurface conditions. The equilibration time may depend on the degree
of soil disturbance during installation, which is influenced by the type of drilling techniques used
to install the soil gas probes (e.g., with more time needed for auger drilling compared with hand
drilling). For example, the California Environmental Protection Agency recommends an
equilibration time of two hours for temporary driven probes and 48 hours for probes installed
using augered borings (CalEPA2012).
EPA recommends documenting wind direction, precipitation information, temperature, and other
site-specific information that can influence soil gas concentration patterns at the time of
sampling, using readily available data sources. These data may be helpful qualitatively in data
interpretation; for example, in reconciling soil gas data collected on multiple occasions or
assessing concordance of sampling data from various media, when not collected
contemporaneously.
EPA recommends that soil gas samples be taken as close to the areas of interest as possible
and preferably from directly beneath the building structure. As vapors are likely to migrate
upward through the coarsest or driest material in the vadose zone, EPA recommends that soil
gas samples be collected from these materials.
Using vertical boring or drilling techniques, it is generally practical to collect soil gas samples
only in locations exterior or adjacentto a building's footprint ("exterior" soil gas samples).
Modeling results for idealized scenarios showthat, in homogeneous soil, soil gas concentrations
tend to be greater beneath the building than at the same depth in adjacent open areas when the
vapor source is underneath the building, even if the source is laterally extensive relative to the
building footprint (e.g., broad plume of contaminated groundwater) (EPA2012b). Given these
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predictions and supporting field evidence (EPA 2012a, see Figure 6; Luoetal. 2009; Patterson
and Davis 2009, see Figure 1), individual exterior soil gas samples cannot generally be
expected to accurately estimate sub-slab or indoorair concentrations. This potential limitation
may be particularly valid for shallow soil gas samples collected exterior or adjacent to a building
footprint. On the other hand, when the subsurface vapor source is not underneath the building,
"exterior" soil gas samples collected from depths belowa building's foundation and along the
side of the building closest to the source may be useful for characterizing a reasonable worst
case condition underneath the building in the absence of routes for preferential vapor migration
or soil gas entry.
Deeper soil gas samples collected in the vadose zone immediately above the source of vapor
contamination (i.e., "near-source" soil gas samples; see Section 6.3.1) can reasonably be
expected to be less susceptible to the diluting effects of ambient air, compared to shallow soil
gas samples. On this basis, deeper soil gas samples collected in the vadose zone immediately
above the source of vapor contamination will tend to be more suitable than will be shallow soil
gas samples for assessing vapor concentrations that may be in contact with the building's sub-
slab.172 Several rounds of sampling are generally recommended to develop an understanding of
temporal variability of "near-source" soil gas concentrations, particularly when these data are
used with the recommended attenuation factor (e.g., a conservative generic value - see Section
6.5.3) to estimate a potential upper-bound indoorair concentration that may arise from vapor
intrusion.173
6.4.5 Groundwater Sampling
Groundwater sampling and analysis also feature prominently in many vapor intrusion
investigations, for example, to help characterize plumes that can serve as vaporsources.
Groundwater sampling methods are not discussed here because practitioners typically are
relatively experienced and trained to collect samples that meet site-specific data quality needs
(see, for example, EPA-ERT2001a). However, Section 6.3.1 provides a few recommended
guidelines for groundwater sampling that are pertinent for characterizing representative vapor
source concentrations forvapor intrusion assessment. One key consideration in sampling
groundwater for vapor intrusion investigations is focusing on characterizing water table
172
Luo et al. (2009) also point to the shortcomings of relying on exterior sampling data, citing significant differences
in vapor concentrations and soil gas composition between interior and exterior sampling locations at a maintenance
warehouse located at a former refinery. They also observed that the spatial variability in the soil-gas distribution was
smaller for soil-gas samples drawn from the source zone, suggesting greater confidence in the assessment of source
zone or "near source" vapor concentrations.
17^
For purposes of this Technical Guide, the term "upper bound indoor air concentration" is intended to be a semi-
quantitative phrase, referring to the high end of the exposure distribution. EPA recommends basing decisions about
whetherto undertake response action for vapor intrusion (i.e., a component of risk management) on a consideration
of a reasonable maximum exposure (e.g., EPA 1989, 1991 a), which is intended to be a semi-quantitative phrase,
referring to the lower portion of the high end of the exposure distribution (see Glossary). Alternatively, a "worst case"
or "reasonable worst case" (see Glossary) indoor air concentration would refer to the upper portion of the exposure
distribution. Section 6.6, which discusses mathematical modeling of vapor intrusion, notes that consideration of a
"worst case" exposure condition may be particularly useful where the predicted "worst case" indoor air concentrations
can be shownto pose acceptable human health risk.
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concentrations. EPA recommends that groundwater samples be taken from wells screened
(preferably overshort intervals) across the top of the water table.174
Groundwater data can be compared to the groundwater VISLs (see Section 6.5).175 When
combined with an appropriate attenuation factor (see Section 6.5.3), groundwater data can be
used to estimate a potential upper-bound indoor air concentration that may arise from vapor
intrusion.176 In these ways, groundwater data can be used to assess the potential for vapor
intrusion from groundwater sources to pose a health concern.
6.4.6 Planning for Building and Property Access
Vapor intrusion investigations generally entail gaining legal access to buildings and properties to
conduct sampling. To address this practical and logistical concern during the planning stage,
EPA recommends that an access agreement be executed between the property owner, any
occupants, and the investigating entity. Section 9.3 provides additional information for
addressing building and property access for sampling.
Obtaining and scheduling access to a property and building can be difficult, whether the
structure is a commercial or institutional building or a private residence. This potential difficulty
can often be overcome by implementing a program of community outreach and engagement
that fosters trust and good relationships. EPA recommends conducting public outreach and
communication for this purpose considering the site-specific community involvement plan (See
Section 9.1).
6.5 Overview of Risk-Based Screening
Risk screening for vapor intrusion generally is performed using site-spedficdata collected via
appropriate methods, as described in Section 6.4. In some cases, pre-existing data identified
during a preliminary analysis can be deemed reliable and adequate for use in risk-based
screening (see Section 5.5).
6.5.1 Objectives of Screening
The primary objective of risk-based screening is to identify sites or buildings unlikely to pose a
health concern through the vapor intrusion pathway. Generally, at properties where subsurface
concentrations of vapor-forming chemicals (e.g., groundwater or "near source" soil gas
concentrations) fall below screening levels (i.e., VISLs), no further action or study is warranted,
so long as the exposure assumptions match those taken into account by the calculations and
the site fulfills the conditions and assumptions of the generic conceptual model underlying the
174 EPA recommends that, to the extent practical, groundwater samples be collected over a narrow interval (e.g., a
few feet or less) just below the water table when the data are to be used for assessing the potential for vapor
intrusion.
175 If available groundwater data do not meet the criteria set forth in Section 6.4.5, the site data review team may
judge whether they are nevertheless representative of potential vapor source concentrations emanating from
groundwater.
EPA recommends basing decisions about whether to undertake response action for vapor intrusion (i.e., a
component of risk management) on a consideration of a reasonable maximum exposure (e.g., EPA 1989, 1991a).
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screening levels. In a similar fashion, the results of risk-based screening can help the data
review team identify areas, buildings, and/orchemicals that can be eliminated from further
assessment.
Subsurface concentrations of vapor-forming chemicals that exceed the VISLforthe respective
medium (e.g., groundwater, soil gas, subslab soil gas) would not automatically trigger mitigation
or subsurface remediation (i.e., they are not offered as response action levels or cleanup
levels). Exceeding a subsurface screening level generally suggests, however, thatfurther
evaluation of the vapor intrusion pathway is appropriate. In this way, risk-based screening,
along with other lines of evidence, can help focus a subsequent site-specific investigation, the
results of which would provide support for considering building mitigation and other risk
management options (see Section 8.0). For example, the results of vapor source strength
screening can help identify and prioritize buildingsforindoortesting.
Finally, risk-based screening can also support:
a preliminary evaluation of human health risk using individual building data (e.g., indoor
air concentrations), which would consider the magnitude of the concentration
exceedance of the indoor air screening level and site-specific risk management
benchmarks (see Section 7.4.1); and
identification of buildings and structures that may warrant prompt action due to potential
explosion threats (see Section 7.5.1).
6.5.2 Scope and Basis for Health-based, Vapor Intrusion Screening Levels
EPA developed VISLs for human health protection that are generally recommended, medium-
specific, risk-based sere en ing-level concentrations intended for use in identifying areas or
buildings that may warrant further investigation of the vapor intrusion pathway. These VISLs are
calculated and documented in the VISL Calculator and are based on:
Current toxicity values selected considering OSWER's hierarchy of sources for toxicity
values (EPA2003).
Physical-chemical parameters for vapor-forming chemicals.
EPA-recommended approaches for human health risk assessment (e.g., EPA2009c,
2014a).
The VISLs for human health protection include indoor air screening levels for long-term (i.e.,
chronic) exposures, which consider the potential for cancer and noncancer effects of vapor-
forming chemicals.177 The VISLs for human health protection also include subsurface screening
levels for comparison to sub-slab soil gas, "near-source" soil gas, and groundwater sampling
177 The VISL Calculator does not include information about radon. Information about characterizing the human health
risk posed by radon can be found on-line at: http://epa-pras.ornl.gov/radionuclides/
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results. These screening levels are derived from the indoor air screening levels for chronic
exposures using medium-specific, generic attenuation factors described further in Section 6.5.3
and AppendixA. The user's guide for the VISL Calculator provides additional information about
derivation of the indoor air and subsurface screening levels (EPA 2015a).
The medium-specific VISLs for human health protection are intended to be compared to:
Building-specific data, such as results from sub-slab soil gas samples, crawl space
samples, or indoor air samples; or
Site- or building-specific data that characterize subsurface vaporsources (e.g.,
groundwater samples, "near-source" soil gas concentrations)
to determine if there is a potential for the vapor intrusion pathway to pose a health concern to
building occupants.
The medium-specific VISLs for health protection are developed considering a generic
conceptual model for vapor intrusion consisting of:
A source of vapors underneath the building(s) either in the vadose zone or in the
uppermost, continuous zone of groundwater.
Vapor migration via diffusion upwards through unsaturated soils from these sources
toward the ground surface and overlying buildings.
Buildings with poured concrete foundations (e.g., basement or slab-on-grade
foundations) that are susceptible to soil gas entry.
A critical assumption for this generic model is that site-specific subsurface characteristics will
tend to reduce or attenuate soil gas concentrations as vapors migrate upward from the source
and into overlying structures. Specific factors that may result in relatively unattenuated or
enhanced transport of vapors into a building include the following:
Significant openings to the subsurface that facilitate soil gas entry into the building (e.g.,
sumps, unlined crawl spaces, earthen floors) otherthan typical utility penetrations.178
Very shallow groundwater sources (e.g., depths to water less than five feet below
foundation level) (see, for example, EPA (2012a), Section 5.2).
Significant routes for preferential, subsurface vapor migration whether naturally-
occurring (e.g., fractured bedrock) or anthropogenic (see Sections 5.4 and 6.3.2).
For purposes of this Technical Guide, the term "significant openings" is intended to refer to forms and amounts of
openings, other than adventitious and intentional openings in a building that are expected to typically be present in all
buildings (e.g., cracks, seams, interstices, and gaps in basement floors and walls or foundations; perforations due to
utility conduits). Such an atypical opening would be "significant" when it is of sufficientvolume and proximity to a
building that it may be reasonably anticipated to influence vapor migration towards or soil gas entry into the building.
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These specific factors are likely to render inappropriate the use of the recommended attenuation
factors and the sub-slab, groundwater, and soil gas VISLs for purposes of identifying sites or
buildings unlikely to pose a health concern through the vapor intrusion pathway. On the other
hand, further evaluation of the vapor intrusion pathway is still appropriate when the sub-slab,
groundwater, and soil gas VISLs are exceeded for samples from a building or site where these
specific factors are present.
Vapor source types that typically make the use of the recommended attenuation factors and
health-based VISLs for groundwaterand soil gas inappropriate include:
Those originating in landfills where methane is generated in sufficient quantities to
induce advective transport in the vadose zone.
Those originating in commercial or industrial settings where vapor-forming chemicals
can be released within an enclosed space and the density of the chemicals' vapor may
result in significant advective transport of the vapors downward through cracks and
openings in floors and into the vadose zone.
Leaking vapors from pressurized gas transmission lines.
In each case, the diffusive transport of vapors may be overridden by advective transport, and
the vapors may be transported in the vadose zone several hundred feet from the source of
contamination with little attenuation in concentration.
In general, EPA recommends considering whetherthe assumptions underlying the generic
conceptual model are attained at a given site. If they are not attained, then EPA recommends
that the medium-specific VISLs not be relied upon as a line of evidence for identifying sites or
buildings unlikely to pose a health concern through the vapor intrusion pathway. Where the
assumptions regarding the subsurface attenuation factors do not or may not apply, EPA
generally recommends collecting indoorair samples.
As noted in Section 6.5.1, these VISLs are not automatically response action levels, although
EPA recommends that similar calculation algorithms be employed to derive cleanup levels (see
Section 7.6). Comparison of sample concentrations to the VISLs is only one factor
recommended for use in determining the need for a response action at a site. As discussed
further in Section 6.5.4, an individual subsurface sampling result that exceeds the respective,
chronic screening level does not establish that vapor intrusion will pose an unacceptable human
health risk to building occupants. Conversely, these generic, single-chemical VISLs do not
account for the cumulative effect of all vapor-forming chemicals that may be present. Thus, if
multiple chemicals that have a common, non-cancer toxic effect are present, a significant health
threat may exist at a specific building or site even if none of the individual substances exceeds
its VISL (see discussion of non-cancer hazard indexin Section 7.4.1).
6.5.3 Recommended Attenuation Factors for Health-based Screening
Vapor attenuation refers to the reduction in volatile chemical concentrations that occurs during
vapor migration in the subsurface, coupled with the dilution that can occur when the vapors
enter a building and mix with indoor air (Johnson and Ettinger 1991). The aggregate effect of
these physical and chemical attenuation mechanisms can be quantified through the use of a
vapor intrusion attenuation factor, which is defined as the ratio of the indoor air concentration
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arising from vapor intrusion to the soil gas concentration at the source or a depth of interest in
the vapor migration route (EPA 2012a).179
EPA compiled a database of empirical attenuation factors for chlorinated VOCs and residential
buildings through review of data from 913 buildings at 41 sites with indoor air concentrations
paired with sub-slab soil gas, groundwater, exteriorsoil gas, or crawl space concentrations
(EPA 2012a). After removing data that do not meet quality criteria and data likely to be
influenced by background sources, the distributions of the remaining attenuation factors were
analyzed graphically and statistically.180 Based upon these analyses, the attenuation factors in
Table 6-1 are recommended by EPA to derive the VISLs for health protection.
With the exception of the "near-source" exteriorsoil gas attenuation factor, the recommended
values for residential buildings are the estimated 95th percentile values, rounded to one
significant figure.181 The rationale for these recommendations and related analyses are provided
in AppendixA. These recommended values are proposed to apply to all vapor-forming
chemicals for use in estimating potential upper-bound concentrations in indoorair that may arise
from vapor intrusion.182 The recommended groundwaterand "near-source" soil gas attenuation
factors do not, however, include the effects of biodegradation.183 On the other hand, because
biodegradation is not expected to occur indoors (i.e., in indoor air in the absence of an air
treatment system), the sub-slab soil gas and crawl space attenuation factors are expected to
apply equally to vapor-forming chemicals that biodegrade in the vadose zone and those that do
not.
As with the medium-specific VISLs, EPA recommends considering whether there are site- or
building-specific factors that may result in unattenuated or enhanced transport of vapors toward
and into a building, such as the presence of preferential migration route(s) as described in
Sections 5.4 and 6.3.2. The presence of such factors is likely to render inappropriate the use of
any of these generic attenuation factors.
179
As defined here, the vapor attenuation factor is an inverse measurement of the overall dilution that occurs as
vapors migrate froma subsurface vapor source into a building; i.e., lower attenuation factorvalues indicate lower
vapor intrusion impacts and greater dilution; higher values indicate greater vapor intrusion impacts and less dilution
(EPA 2012a, b). Johnson and Ettinger (1991) utilized the symbol a forthe vapor intrusion attenuation factor. For
example, the subslabsoil gas attenuation factor is intended to account for concentration dilution arising during
migration through openings in the foundation and from mixing of subsurface contaminants inside the building. The
groundwater attenuation factor is intended to account for concentration dilution arising during vapor migration from
the groundwater table through the vadose zone, in addition to concentration dilution arising during migration through
openings in the foundation and from mixing of subsurface contaminants inside the building.
180
A summary of the resulting distributions is provided in Appendix A of this document.
181
The recommended "near-source" exterior soil gas attenuation factor corresponds to approximately the estimated
75 percentile value.
189
EPA recommends basing decisions about whether to undertake response action for vapor intrusion (i.e., a
component of risk management) on a consideration of a reasonable maximum exposure (e.g., EPA 1989, 1991a).
18^
Appropriate data can be collected and evaluated, as described in Section 6.3.2, to characterize and document the
occurrence of biodegradation in the vadose zone and its effects in attenuating vapor concentrations of biodegradable
vapor-forming chemicals.
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TABLE 6-1
RECOMMENDED VAPOR ATTENUATION FACTORS FOR RISK-BASED
SCREENING OF THE VAPOR INTRUSION PATHWAY184
Sampling Medium
Groundwater, generic value, except for shallow
water tables (less than five feet below foundation) or
presence of preferential vapor migration routes in
vadose zone soils
Groundwater, specific value for fine-grained vadose
zone soils, when laterally extensive layers are
present185
Sub-slab soil gas, generic value
"Near-source" exterior soil gas, generic value
except for sources in the vadose zone (less than five
feet below foundation) or presence of routes for
preferential vapor migration in vadose zone soils
Crawl space air, generic value
Medium-specific Attenuation Factor for
Residential Buildings
1E-03 (0.001)
5E-04 (0.0005)
3E-02 (0.03)
3E-02 (0.03)
1E-00(1.0)
The VISL Calculator (http://vwwv.epa.gov/oswer/vaporintrusion/guidance.html) also facilitates
calculation of groundwater screening levels based on the recommended attenuation factor for
fine-grained soil. EPA recommends that any use and application of this semi-site-specific
groundwater attenuation factor be supported by site-specific geologic information (i.e., site
geology and subsurface lithology). Significant characterization of the vadose zone may be
needed to demonstrate that fine-grained layers are laterally extensive overdistancesthat are
large compared to the size of the building(s) or the extent of vapor contamination at a specific
site, which is the recommended support for using the semi-site-specific attenuation factorfor
184 Use of these attenuation factors for estimating indoor air concentrations is contingent upon site conditions fitting
the generic model of vapor intrusion described in Section 6.5.2 and subsurface conditions being characterized
considering the recommendations in Sections 6.3 and 6.4.
185 The Draft VI Guidance allowed for the modification of VISLs for groundwater by incorporating a lower attenuation
factor, based upon "some site-specific inputs", which estimates a greater reduction in vapor concentrations in the
vadose zone than the generic value (EPA 2002c, 201 Ob). In the Draft VI Guidance, graphs were provided from which
such "semi-site-specific" attenuation factors could be selected and justified based upon site-specific soil type and
depth to the watertable. Based upon analysis of EPA's expanded database, a single groundwater attenuation factor
is provided in this Technical Guide for fine-grained soils.
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fine-grained soil.186 For purposes of applying the groundwater attenuation factors, EPA
recommends the depth to groundwater be estimated relative to the bottom of the building
foundation and be based upon the seasonal high groundwater table.
6.5.4 Comparing Sample Concentrations to Health-based Screening Levels
When evaluating environmental sampling results to assess the vapor intrusion pathway, it is
important to first determine that the samples were collected appropriately. Section 6.4 provides
information about recommended sampling locations and procedures for vapor intrusion
investigations. In addition, EPA recommends collecting and evaluating appropriate site-spedfic
information to demonstrate that the property fulfills the conditions and assumptions of the
generic conceptual model underlying the VISLs, as described in Section 6.5.2.
After verifying that the CSM justifies the use of the VISLs, the individual sample concentrations
may be compared to the appropriate medium-specific screening levels. In order to select the
appropriate target media concentrations for comparison, it generally is important to identify
whether a source of vapors for a building or a developed area occurs in the unsaturated zone,
which is an important aspect of the CSM. This allows the site data to be segregated into two
categories:
Data representing areas where contaminated groundwater is the only source of
contaminant vapors.
In this first case, groundwater VISLs are generally appropriate to use to evaluate
groundwater concentrations (also see sampling recommendations in Sections 6.3.1 and
6.4.5). Under these circumstances, EPA recommends that the plume be shown to be
stable or shrinking (i.e., is not migrating or rising in concentration, including hazardous
byproducts of any biodegradation) to establish that the potential for vapor intrusion to
pose a human health risk from vapor intrusion will not increase in the future. "Near-
source" soil gas data (i.e., soil gas samples collected immediately above the water table)
could also be compared to the soil gas VISLs to obtain a corroborating line of evidence
(see recommendations in Section 6.3.1).
When the anticipated outcome of the screening is a finding that groundwater poses
acceptable human health risk from vapor intrusion on an area-wide basis, it may be
appropriate to compare sampling results for the most greatly impacted well within the
area of interest and showthat these results are less than the groundwater VISLs.
Data representing areas where the underlying vadose zone soil contains a source of
vapors (e.g., residual NAPL).
In this second case, EPA recommends that only soil gas VISLs be used and compared
to results from "near-source" soil gas samples collected nearthe vapor source zone
The general soil type assigned to paired vapor intrusion data in the EPA's database "generally represents the
coarsest soil described in the vadose zone near the sample location" unless "sufficientstratigraphic information was
available to indicate finer sediments are laterally continuous" (EPA 2012a). EPA recommends that similar criteria be
applied to justifying the use of the semi-site-specific attenuation factor for groundwater (or selection of soil-related
parameters for modeling); see Section 6.6. For these purposes, soil classified as clay, silty clay, silty clay loam, or silt
consistent with the U.S. Soil Conservation Service classification system can be considered to be "fine-grained."
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(also see sampling recommendations in Sections 6.3.1 and 6.4.4). In this situation, the
groundwaterVISLs (and vapor attenuation factors for groundwater) are not
recommended for estimating potential upper-bound indoor air concentrations, because
they have been derived assuming no other vapor sources exist between the water table
and the building foundation.
In both cases, because of the complexity of the vapor intrusion pathway, EPA recommends that
professional judgment be used when applying the VISLs.
Generally, if all subsurface sample concentrations for a given building or area are less than the
respective medium-specific screening level, then vapor intrusion is less likely to pose an
unacceptable human health risk to building occupants. On the other hand, when individual
sample concentrations exceed the respective screening level, additional assessments may be
warranted. So, for example, if a groundwater or "near-source" soil gas concentration exceeds
the respective screening level, it is recommended that sub-slab soil gas testing and indoorair
testing be conducted.
However, we would note that any individual subsurface sampling result that exceeds the
respective, chronic screening level does not establish that vapor intrusion will pose an
unacceptable human health risk to building occupants. Forone, the subsurface screening levels
are expected to be conservative (i.e., are likely to over-estimate the contribution to indoorair
levels arising from vapor intrusion) for many buildings due to the use of a high-end attenuation
factor (see Section 6.5.3). In many cases, indoor air concentrations arising from vapor intrusion
would be expected to be lower than those estimated using the recommended generic
attenuation factors. For carcinogens, the screening levels are set using a one-per-million lifetime
cancer risk (i.e., 10"6), whereas EPA recommends consideration of a cancer risk range when
making risk management decisions (see Section 7.4.1). Finally, sampling results can be
expected to be variable spatially and temporally and these screening levels assume a long
period of exposure at the stated concentration.
Owing to the temporal variability in building-specific data and the potential temporal and spatial
variability in soil gas vapor concentrations, EPA generally recommends multiple samples be
collected (see Section 6.4) and compared to the respective medium-specific screening level. In
addition, the results of risk-based screening are generally most useful when they can be
evaluated for indoor air and subsurface vapor sources concurrently and in the context of the
CSM. EPA, therefore, generally recommends that multiple lines of evidence be developed and
their results weighed togetherwhen evaluating and making risk-informed decisions pertaining to
vapor intrusion. EPA generally recommends that concordance among the multiple lines of
evidence be obtained, particularly when considering a determination that the vapor intrusion
pathway is incomplete or does not pose an unacceptable human health risk. Sections 7.1, 7.2,
and 7.3 provide additional information and recommendations about developing and using
multiple lines of evidence and risk management decision-making.
6.5.5 Planning for Communication of Sampling Results
EPA recommends the community involvement or public participation plan (See Section 9.1)
describe and address community questions, concerns, and preferences for participation
regarding sampling results. Generally, EPA recommends that the site planning team provide
validated results to property owners and occupants. These results can be transmitted to
relevant parties in a letter, along with a description of what future actions, if any, may be
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warranted. In addition, the site planning team may choose to hold a community meeting to
discuss the sampling results in general terms and EPA's plans, if any, for response actions.
Section 9.4 provides additional information for communicating sampling results.
6.6 General Principles and Recommendations for Mathematical Modeling
When suitably constructed, documented, and verified, mathematical models can provide an
acceptable line of evidence supporting risk management decisions pertaining to vapor intrusion.
In certain situations (e.g., forfuture construction on vacant properties), it is particularly useful to
employ mathematical modeling to predict reasonable maximum indoor air concentrations,
because indoor air testing is not possible.
Mathematical modeling is most appropriately used in conjunction with other lines of evidence.
For example, in the brownfield development case (i.e., yet-to-be-constructed building), EPA
generally recommends these additional lines of evidence include, at a minimum, data that
characterize potential subsurface vapor sources and assodated geologic and hydrologic
conditions in the vadose zone (see Sections 6.3.1 and 6.3.2).
Generally, mathematical models transform empirical values of input parameters into predictions
of chemical concentrations in environmental media. The model input parameters are equally as
important to the results as the mathematical components of the model (i.e., governing equations
and solution algorithms). As a consequence, the results critically depend on the choices for the
inputs.
Historically, to assure confidence in predictions of mathematical models, they have been
compared to measured, site-specific values. When measured and predicted values do not
reasonably match, model input parameters are adjusted through calibration. For example,
calibration is commonly used in groundwater flow modeling, in which model-predicted
groundwater levels are matched to measured groundwater levels for a baseline condition to
gain insight into hydrogeologic properties. The calibrated input parameters must reasonably
represent the underlying phenomena and the characteristics of the model must reasonably
match the field situation. Calibration of mathematical models is known to be non-unique, so that
different sets of parameters can be used to fit the same observed data. This means that
calibration does not produce a theoretically correct set of parameters. Because various values
of input parameters could be used in the calibrated model, there will always be uncertainty as to
the actual values.
Three approaches exist for applying mathematical models in these circumstances:
1) Calibrating the mathematical model to the measured indoor air concentration (and,
possibly, the sub-slab soil gas concentration) considered to be representative of vapor
intrusion (i.e., background vaporsources have been identified and removed prior to
sampling and data evaluation indicates that the concentration is reasonably attributable
to vapor intrusion). Calibration entails adjusting the input parameters within plausible and
realistic ranges so that the predicted indoor air concentrations (or sub-slab soil gas
concentrations) are similar to the measured vapor concentrations. The adjusted input
parameters can then be compared to site-specific conditions and data to verify that the
CSM and calibrated model are coherent and sound.
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2) Conducting an uncertainty analysis (perhaps using an automated uncertainty analysis
(see http://vwwv.epa.gov/athens/leam2model/part-two/onsite/uncertaintv-vi.html as only
one example)) to understand where, within the probability distribution of results, model
results with pre-selected default parameters lie. This approach may be particularly useful
where indoor air concentrations have not been measured or non-site-specific inputs
have been used.
3) Using a bounding case analysis, where parameters are chosen to represent conditions
that give a high-impact (e.g., "reasonable worst") case - see Glossary - or "worst"
(maximum plausible)187 case. This approach may be particularly useful where the
predicted indoor air concentrations for the bounding case can be shown to pose
acceptable human health risk.188 The range of predicted indoor air concentrations can be
established if the analysis also includes a low-impact ("best") case.
Unless site-specific parametervalues are obtained for input parameters and the mathematical
model is calibrated to field data, use of default input parameter values will generate model
results that lie at an unknown point within an uncertainty band of the model outcomes. Because
the combined effect of parameter uncertainty is large, a one- or two-order of magnitude error
might be made unknowingly. To reduce these errors, sub-slab vapor sampling could be used to
characterize the vapor concentration(s) beneath a building. Model results (i.e., predicted sub-
slab soil gas concentrations) that match measured values would have increased confidence.
Alternately, using bounding estimates of parametervalues could provide a conservative model
result that would be expected to represent the reasonable worst case of potential exposure.
Three examples followwhere differing applications of mathematical models would be useful in
vapor intrusion assessment:
Verify General Magnitude. Modeling using site-specific inputs can be useful for verifying
the general magnitude of measured indoor air sample concentrations, which may allow
risk managers to reach supportable condusions not to conductadditional indoorair
testing. In this situation, the model could be calibrated to indoorair measurements and
the plausibility of the calibrated input parameters evaluated. If the calibrated model input
parameters are plausible, then they can be considered an additional line of evidence
supporting risk managementdecisions.
Explore Range of Outcomes through Uncertainty Analysis. In certain situations, indoor
airtesting is not possible (e.g., forfuture construction on vacant properties) orfeasible.
For purposes of this Technical Guide, the phrase "worst case indoor air concentration" is intended to be a semi-
quantitative phrase, referring to the high end of the exposure distribution. "No-further-action" decisions can normally
be supported more confidently when the "worst case indoor air concentration" can be shown to pose acceptable
health risks. Under these conditions, the "reasonable maximum exposure" (see Glossary) typically would also pose
acceptable health risks.
"Bounding estimates" purposely overestimate the exposure or dose in an actual population forthe purpose of
developing a statement that the risk is "not greater than..." (EPA 1992c).
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Here the range of possible outcomes could be explored with the model through an
uncertainty analysis. For example, model input parameters, including building and
vadose zone soil properties, could be varied within plausible ranges to determine the
parameters to which the model is most sensitive to guide field investigations. Uncertainty
analyses can also be used to ascertain whether the subsurface vapor source
concentrations are such that indoor air samples would not be expected to contain
detectable levels of vapor-forming chemicals arising from vapor intrusion.
Generate Bounding Estimates. If the range of parameter values is known with
confidence for the site, then parameters can be chosen to represent the bounding case
of maximum plausible vapor intrusion (i.e., worst case).
In each of these examples, model parameters might vary in space and time because of
subsurface heterogeneity, transient hydrologic conditions, or variation in building operation.
Thus, there is a need for characterizing spatial and temporal variability.
Mathematical models provide opportunities to predict conditions that cannot be observed
directly, but the reliability of the results need to be confirmed, especially when limited site-
specific data are available and the model is not calibrated to observed indoor air concentrations.
Use of a generic, conservative attenuation factor (see Section 6.5.3) to predict potential, upper-
bound indoor air concentrations (based upon soil gas concentrations - see Sections 6.4.3 and
6.4.4) implicitly represents use of a mathematical model, even when the attenuation factor is
selected from an empirical data set. Whetherthe mathematical model is implicit (e.g., generic,
conservative attenuation factor) or explicit (e.g., mathematical model that generates a bounding
estimate), both analytic approaches make the assumption that site-specific attenuation is likely
to be greater and the indoor air concentration(s) is (are) likely to be lower than predicted
value(s).
The use of extreme and non-representative assumptions or parametervalues is the most
common weakness of mathematical modeling for environmental assessments. Mathematical
modeling typically yields more reliable results when used with high-quality, site-specific data
inputs (that is, representative groundwater or soil gas concentrations, depth to groundwater, soil
type and moisture content underneath the building, and the building conditions (e.g., air
exchange rate, building mixing height), for example); in these cases, the site-specific data inputs
and CSM provide additional lines of evidence supporting the use of mathematical modeling as a
line of evidence.
Whenever mathematical modeling is used to make predictions pertaining to vapor intrusion,
EPA recommends that the site planning and data team:
Identify the underlying mathematical model and include appropriate references to
document that it has been peer-reviewed.
Verify that the selected model fits the CSM and is appropriate for the chosen purpose.
Document all inputs and outputs in a readily recognizable and understandable format.
Identify the critical parameters and conduct a sensitivity analysis forthe most critical
parameters.
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Determine and document the appropriate modeling approach (e.g., calibration,
uncertainty analysis, bounding case analysis).
Perform new individual measurements (i.e., field sampling) to confirm one or more
results of the modeling.
A critical assumption underlying almost all mathematical models of vapor intrusion is that site-
specific subsurface characteristics will tend to reduce or attenuate soil gas concentrations as
vapors migrate upward from the source and into overlying structures. Mathematical modeling of
vapor intrusion is, therefore, not generally recommended for sites and buildings where
unattenuated or enhanced transport of vapors toward and into a building is reasonably
expected. Sections 5.4, 6.3.2, and 6.5.2 identify several factors that may result in unattenuated
or enhanced transport of vapors toward and into a building.
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7.0 RISK ASSESSMENT AND MANAGEMENT FRAMEWORK
This section provides general recommendations about risk-informed decision-making pertaining
to vapor intrusion. The risk management information described herein presumes thata sound
CSM has been developed (see Sections 5.4 and 6.3), which is supported by multiple lines of
evidence, and that subsurface vapor sources have been characterized (see Section 6.3.1)
sufficiently to support the risk management decisions forthe site. EPA also notes that temporal
and spatial variability of sampling data can span at least an order of magnitude and often more.
Site-specific decisions potentially supported by the information described in this section include:
Whether to install engineered exposure controls to prevent or reduce the impacts of
vapor intrusion in specific buildings.
Whether to remediate subsurface vapor sourcesfor the site to reduce risks posed by
vapor intrusion.
Whether the vapor intrusion pathway is incomplete and there is no potential for
unacceptable human exposure under current or future conditions.
Whether to collect additional information as part of the detailed vapor intrusion
investigation or monitor indoor air as part of an overall vapor intrusion remedy.
As conditions warrant and resources allow, EPA generally recommends that officials
responsible for overseeing cleanups pursuantto RCRA and CERCLA ensure that past
decisions pertaining to vapor intrusion continue to be supported by current conditions (EPA
2002b).
Finally, EPA encourages systematic approaches to decision-making, which can foster scientific
rigor, consistency, and transparency.
7.1 Collect Site-specific Lines of Evidence
Current practice suggests thatthe vapor intrusion pathway generally be assessed using multiple
lines of evidence. As discussed in Sections 5.1, 5.4, 5.5.2, 6.3, 6.4, and 6.5, appropriate lines of
evidence to support development of the CSM and evaluate the vapor intrusion pathway may
include, but are not limited to:
Subsurface Vapor Sources
Site history and source of the contaminants to demonstrate that vapor-forming chemicals
have been or may have been released to the underlying and surrounding subsurface
environment and identify the type of vapor source (e.g., vapor-forming chemicals
dissolved in groundwater or present in a NAPL).
Groundwater data (generally recommended from more than one sampling event), as
appropriate, to confirm the presence of a water-table aquifer, if present, as a source of
vapors and establish its chemical and hydrogeologic characteristics.
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Soil gas data, bulk soil sampling data,189 and/or NAPL sampling data to confirm the
presence of contamination in the vadose zone, if present, as a source of vapors and
establish its chemical and physical characteristics.
Sub-slab (or crawl space) soil gas data to assess concentrations potentially available for
entry with any intruding soil gas (generally recommended from multiple sampling events
and in multiple locations to reduce the chance of reaching a false-negative conclusion
(i.e., concluding subsurface vapor source strength is limited when vapor intrusion
actually poses an unacceptable human health risk) or a false-positive conclusion.
Comparison of groundwater and/or soil gas concentrations to VISLs to evaluate source
strength and potential for a health concern if the vapor intrusion pathway is complete.
Vapor Migration and Attenuation in the Vadose Zone
Soil gas survey data, including some level of vertical and spatial profiling, as appropriate,
to confirm soil gas migration and attenuation along anticipated routes in the vadose zone
between sources and buildings.
Data on site geology and hydrology (e.g., soil moisture and porosity) to support the
interpretation of soil gas profiles, the characterization of gas permeability, and the
identification of anticipated soil gas migration routes in the vadose zone or the
identification and characterization of impeded migration.
Vertical profiles of chemical vapors, electron acceptors for microbial transformations
(e.g., oxygen), and degradation products (e.g., methane, vinyl chloride) to characterize
attenuation due to biochemical (e.g., biodegradation) processes.
Utility corridor assessment to identify preferential migration routes, if any, that facilitate
subsurface vapor migration between sources and towards and into buildings
Building Foundation Assessment, Including Susceptibility to Soil Gas Entry
Building construction and current conditions, including utility conduits or other
preferential routes or openings for soil gas entry, heating and cooling systems in use,
and any segmentation of ventilation and air handling.
Instrumental (e.g., PID) readings to locate and identify potential openings for soil gas
entry into buildings.
As noted in Section 6.4, bulk soil sampling and analysis can be used to characterize the chemical composition
and general location of contamination; for example, high soil concentrations generally would indicate impacted soil.
On the other hand, non-detect results for soil samples cannot be interpreted to indicate the absence of a subsurface
vapor source, because of the potential for vapor loss due to volatilization during soil sampling, preservation, and
chemical analysis. Therefore, bulk soil (as opposed to soil gas) sampling and analysis is not currently recommended
for estimating the potential forvapor intrusion to pose unacceptable human health riskin indoor air.
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Grab samples of soil gas or indoor air near openings to characterize the composition of
presumptive soil gas entering buildings.
Pressure data to assess the driving force for soil gas entry into building(s) via advection.
Tracer-release data to verify openings in building foundations for soil gas entry or assess
fresh air exchange within buildings.
Interior Assessment
Indoor air sampling data (Section 6.4.1) to assess the presence of subsurface
contaminants in indoor air (Section 6.3.4), estimate potential exposure levels to building
occupants to support site-spedfic human exposure and human health risk assessments
(see Section 7.4), and otherwise diagnose vapor intrusion and characterize background
concentrations (Section 6.3.5).19ฐ
Indoor and Outdoor Sources of Vapor-forming Chemicals Found in the Subsurface
Building-specific indoorsources of volatile chemicals (Section 2.7).
Concurrent outdoorair data to assess potential contributions of ambient air to indoorair
concentrations (Sections 6.3.5 and 6.4.2).
Comparative evaluations of indoor air and sub-slab soil gas data (e.g., Section 6.3.5),
including calculation and comparison of building-specific, empirical attenuation factors
(EPA 2012a, Section 3.0) (e.g., to assess their consistency among subsurface
contaminants to assist in identifying indoor vapors arising from vapor intrusion).
Additional Supporting Lines
Results of statistical analyses (e.g., data trends, contaminant ratios) to support data
interpretation.
Results of mathematical modeling that rely upon site-specific inputs (Section 6.6).
The relative utility of these and other individual lines of evidence will depend on site-specific
factors, as described and documented in the CSM (Section 5.4), and the objectives of the
investigation (Section 6.3). For example:
When the primary subsurface vapor source is residual NAPL in the vadose zone, bulk
soil data would typically be collected to characterize the chemical composition and
general location of contamination; forexample, high soil concentrations generally would
190
In certain cases, depending in part on the results (e.g., concentrations exceed risk-based screening levels), indoor
air sampling data may be a sufficient basis for supporting decisions and recommendations to undertake pre-emptive
mitigation (see Sections 3.3 and 7.8) in lieu of additional rounds of sampling and analysis or an evaluation of the
contribution of background sources to indoor air concentrations.
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indicate impacted soil in the vadose zone, as discussed in Sections 6.3.1 and 6.4.191 In
this situation, "near source" soil gas data, ratherthan groundwaterdata, would be
recommended for assessing the potential for vapor intrusion to pose an unacceptable
human health risk to occupants of any building overlying the NAPL zone. On the other
hand, when the subsurface vaporsource underneath a building is shallowgroundwater,
groundwater sampling data from the uppermost hydrogeologic unit would be an
appropriate line of evidence for purposes of assessing the potential for vapor intrusion to
pose an unacceptable human health risk, unlike the previous example.
In both of the preceding cases, information about the soil conditions (e.g., soil type and
moisture) underlying the buildings would be useful for characterizing the subsurface
vapor migration route between the subsurface vapor source and the building. Sub-slab
soil gas samples and indoor air samples (if background sources are removed or
accounted for), in concert with other lines of evidence, can provide a strong line of
evidence regarding whether the vapor intrusion pathway is complete.
For an industrial building, indoor air testing while the HVAC system is not operating (see
Section 6.3.3) could be useful for diagnosing vapor intrusion. On the otherhand, single-
family detached homes can generally be presumed susceptible to soil gas entry when
heating or cooling systems are operating.
7.2 Weigh and Assess Concordance Among the Lines ofEvidence
To the risk manager, the ideal outcome from collecting multiple lines of appropriate evidence is
a concordant set of site-specific information that unambiguously supports decisions that can be
made confidently. However, based upon observations at many buildings and sites, the vapor
intrusion site where all available information is in agreement and is unambiguous may be the
exception ratherthan the rule. Some lines of evidence may not be definitive (e.g., indoor air and
subsurface concentrations can be greatly variable temporally and spatially). At worse, some
individual lines of evidence may be inconsistent with other lines of evidence. In general, when
lines of evidence are not concordant and the weight of evidence does not support a confident
decision, EPA recommends re-evaluating the CSM, which may warrant adjusting the CSM to
better represent the weight of the available evidence.
For example, a building overlying contaminated shallow groundwater may have high
concentrations of vapor-forming chemicals in the sub-slab soil gas samples, but lower
concentrations in soil gas samples collected exterior to the building at intermediate depths.
In this example, the exterior soil gas data suggest there may not be a connected vapor
migration path between the groundwater source and the building that exhibits continuous
attenuation along the path. Nevertheless, the data review team may conclude that vapor
migration is capable of transporting hazardous vapors from the source to building(s) if the
groundwater and sub-slab soil gas samples share common contaminants that are known or
suspected to have been released at the site (for example, samples of both groundwater and
191
Because of the large uncertainties associated with measuring concentrations of volatile contaminants introduced
during soil sampling, preservation, and chemical analysis, bulk soil (as opposed to soil gas) sampling and analysis is
not currently recommended for estimating the potential for vapor intrusion to pose unacceptable human health riskin
indoor air. In addition, there are uncertainties associated with soil partitioning calculations.
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the sub-slab soil gas contain TCE). In this circumstance, the data reviewteam may wish to
consider whether the occurrence of a higher TCE concentration in the sub-slab soil gas than
in the exterior soil gas sample(s) can be explained by: (1) a previously unknown or
unrecognized utility corridor or other preferential migration route that provides relatively
unattenuated vapor transport between the groundwater and the building; (2) a previously
unknown or unrecognized source of TCE in the vadose zone; or (3) the possibility that the
exterior soil gas samples were not well located for purposes of characterizing subsurface
vapor migration. This example also underscores the importance of developing an adequate
CSM (e.g., identify all sources and preferential routes of subsurface vapor migration) and
illustrates why EPA generally recommends that the vapor intrusion pathway not be deemed
incomplete based upon any single line of evidence (EPA 2010b), such as exterior soil gas in
this example.
When lines of evidence are not concordant and the weight of evidence does not support a
confident decision, it may also be appropriate to collect additional lines of evidence, possibly
including additional samples, depending upon the CSM. For example:
Appropriate site-specific testing (see Section 6.3.5) can be conducted to assess the
contribution of background sources of vapor-forming chemicals, including comparisons
among chemicals of their relative concentrations in indoor air, outdoor air, and soil gas.
Background sources of vapor-forming chemicals may help to explain situations where
the indoor air concentration is higherthan can be accounted for by the subsurface vapor
source or the sub-slab soil gas data.
Diagnostic testing of indoor air (see Section 6.4.1), building condition assessments or
utility surveys, or supplemental hydrogeologiccharacterization (see Section 6.3.2) can
be used to investigate the suspected presence of preferential migration routes, such as
those described in Sections 5.4 and 6.3.2. Such investigations may help to explain
situations where the sub-slab or indoor air concentration appears to reflect unattenuated
vapor transport from the subsurface vapor source.
Building susceptibility to vapor intrusion can be tested (see Section 6.3.3), which may
help to explain situations where the indoor air concentration is significantly lower than
expected based upon the sub-slab soil gas data.
Vapor migration in the vadose zone can be further characterized to identify impedances
to vapor migration (see Section 6.3.2), appropriate semi-site specific attenuation factors
can be considered (see Section 6.5.3), and appropriate modeling can be conducted (see
Section 6.6) to investigate site-specific vapor attenuation. Such data and analyses may
help to explain situations where the sub-slab soil gas concentration is significantly lower
than expected based upon groundwatersource or "near-source" soil gas concentrations
and the respective medium-specific attenuation factor (Section 6.5.2 and AppendixA). In
some of these situations, the vapor intrusion pathway may be impeded, or perhaps even
incomplete, due to geologic, hydrologic, or microbial characteristics in the vadose zone
(see Sections 6.3.2 and 7.3).
Recognizing the temporal and spatial variability of indoor air and subsurface concentrations and
the potentially episodic nature of vapor intrusion at some sites (Section 2), EPA generally
recommends collecting multiple rounds of sampling in the respective media from multiple
locations (see Section 6.4) to reduce the chance of reaching a false-negative or false-positive
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conclusion. Considerable judgment may be necessary when evaluating multiple data sets from
individual sampling events to supportdecision-making.
In summary, EPA recommends the appropriate use and evaluation ("weighing") of multiple lines
of evidence for determining whether the vapor intrusion pathway is complete or not, whether
any elevated levels of contaminants in indoor air are likely caused by subsurface vapor intrusion
versus an indoor source or an ambient (outdoor) air source, whether concentrations of
subsurface contaminants in indoorair may pose a health concern, and whether interim
response measures to mitigate vapor intrusion are warranted.
7.3 Evaluate Whether the Vapor Intrusion Pathway is Complete or Incomplete
For purposes of this Technical Guide, and as reflected in the conceptual model of vapor
intrusion (see Section 2), the vapor intrusion pathway is referred to as "complete" for a specific
building or collection of buildings when the following five conditions are met undercurrent
conditions:
1) A subsurface source of vapor-forming chemicals is present underneath or nearthe
building(s) (see Sections 2.1, 5.3,6.2.1, and 6.3.1);
2) Vapors form and have a route along which to migrate (be transported) toward the
building(s) (see Sections 2.2 and 6.3.2);
3) The buildings are susceptible to soil gas entry, which means openings exist for the
vapors to enter the building and driving 'forces' exist to drawthe vapors from the
subsurface through the openings into the building(s) (see Sections 2.3 and 6.3.3);
4) One or more vapor-forming chemicals comprising the subsurface vapor source(s) is (or
are) present in the indoor environment (see Sections 6.3.4 and 6.4.1); and
5) The building is occupied by one or more individuals when the vapor-forming chemical(s)
is (or are) present indoors.
Considerable scientific and professional judgment will likely be needed when weighing lines of
evidence to determine whether the vapor intrusion pathway is complete or incomplete. Each of
the first four conditions generally entails obtaining and weighing multiple lines of evidence,
whereas the fifth condition generally can be confidently determined by direct observation. EPA
recommends considering and evaluating togetherthe various lines of evidence in determining
completeness of the vapor intrusion pathway undercurrent conditions.
As noted previously (e.g., Section 3.2), EPA recommends that risk management decisions also
consider whether the vapor intrusion pathway is 'potentially complete' under reasonably
expected future conditions. The vapor intrusion pathway is referred to as 'potentially complete'
for a building when:
a subsurface source of vapor-forming chemicals is present underneath or near an
existing building or a building that is reasonably expected to be constructed in the future;
vapors can form from this source(s) and have a route along which to migrate (be
transported) toward the building; and
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three additional conditions are reasonably expected to all be met in the future, which
may not all be met currently; i.e.,
o the building is susceptible to soil gas entry, which means openings exist for the
vapors to enter the building and driving forces exist to draw the vapors from the
subsurface through the openings into the building;
o one or more vapor-forming chemicals comprising the subsurface vaporsource(s)
is (or will be) present in the indoor environment (see Sections 6.3.4 and 6.4.1);
and
o the building is or will be occupied by one or more individuals when the vapor-
forming chemical(s) is (or are) present indoors.
This determination also generally entails obtaining and weighing multiple lines of evidence.
A complete pathway indicates that there is an opportunity for human exposure, which warrants
further analysis to determine whether there is a basis for undertaking a response action(s).
Specifically, a complete exposure pathway does not necessarily mean that an unacceptable
human health risk exists due to vapor intrusion. Rather, specific exposure conditions, such as
the magnitude, frequency, and duration of exposures, and the contribution from background
concentrations warrant examination; hence, EPA recommends additional analyses be
conducted to assess and characterize human health risk to building occupants where the vapor
intrusion pathway is determined to be complete (see, for example, Sections 7.4 and 6.3.5). On
the other hand, human exposure, and hence human health risk, from the vapor intrusion
pathway would not exist if the pathway is incomplete.
The conceptual model described in Section 2 identifies the characteristics of the vadosezone
that could render the vapor intrusion pathway incomplete undercurrent and future conditions.
These individual characteristics include, but are not limited to:
Soil layers that significantly and persistently impede vaportransportdue to geologic or
hydrologic conditions (e.g..fine-grained soil, soil with high moisture content) and are
laterally extensive over distances that are large compared to the size of the building(s)
or the extent of subsurface contamination with vapor-forming chemicals; and
A biologically active vadose zone that can significantly and persistently attenuate soil
gas concentrations due to biodegradation, in which all appropriate conditions (e.g.,
nutrients, moisture, and electron acceptors, such as dissolved oxygen in the case of
aerobic biodegradation) are readily available overa laterally extensive area.
EPA recommends demonstrating these characteristics, when present, by collecting, evaluating,
and documenting multiple lines of evidence, as identified in Section 6.3.2. In addition, EPA
recommends that any determination that the vapor intrusion pathway is incomplete be
supported by site-specific evidence to demonstrate that:
The nature and extent of vapor-forming chemical contamination in the subsurface has
been well characterized, as discussed in Sections 6.3.1 and 6.4. Ideally, where
groundwater is the source of vapors, the plume has been shown to be stable or
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shrinking to establish that the potential for vapor intrusion to pose a health concern will
not increase in the future.
The types of vapor sources and the conditions of the vadose zone and surrounding
infrastructure do not present opportunities forunattenuated or enhanced transport of
vapors toward and into any building (e.g., via a preferential migration route(s)), as
discussed in Sections 5.4, 6.2.1, 6.3.2, and 6.5.2.
When the vapor intrusion pathway is determined to be incomplete, then vapor intrusion
mitigation is not generally warranted undercurrent conditions. EPA recommends that site
managers also evaluate whether subsurface vapor sources that remain have the potential to
pose a complete vapor intrusion pathway and unacceptable human health risk due to vapor
intrusion in the future if site conditions were to change. Forexample, potentially unpredictable
changes in the transitory soil characteristics (e.g., soil moisture) and soil gas concentrations
may occur as a result of constructing a new building or supporting infrastructure. Eithertype of
change could result in the potential for unacceptable human health risk due to vapor intrusion in
the future.
Response actions may, therefore, be warranted to protect human health wherever and as long
as subsurface vapor sources remain that have the potential to pose unacceptable human health
risk in the future due to vapor intrusion. These response actions (see Section 7.7) may include
institutional controls (seeSection 8.6) (e.g., to record and alert parties aboutthe presence of
subsurface vaporsourcesand/orto inform the need for a confirmatory vapor intrusion
investigation in case infrastructure or geologic conditions are modified in the future). In addition,
subsurface remediation may be warranted to protect human health or the environment via other
exposure pathways (e.g., groundwater discharge to surface water bodies), consistent with
applicable statutes and considering EPA guidance.
7.4 Conduct and Interpret Human Health Risk Assessment
EPA generally recommends that a human health risk assessment be conducted to determine
whether the potential human health risk posed to building occupants by a complete or
potentially complete vapor intrusion pathway are within or exceed acceptable levels, consistent
with applicable statutes192 and considering EPA guidance (EPA 1991 a, 2009c). The primary
purpose of this risk assessment is to provide risk managers with an understanding of the actual
and potential risks to human health posed by vapor intrusion under current and reasonably
expected future conditions. This information may be useful in determining whethera currentor
potential future threat to human health exists, as described in Sections 7.4.1, 7.4.2, and 7.5.2,193
which warrants response action(s), as described in Sections 7.7 and 8.
192
In the RCRA corrective action program, any human health risk assessment would typically be conducted during
the RCRA facility investigation, if a release to the environment is identified. Under the CERCLA remedial program, a
human health risk assessment would typically be conducted during the remedial investigation and is generally
referred to as a baseline (i.e., pre-cleanup) risk assessment.
mo
In appropriate circumstances (e.g., where time is of the essence to ensure protection of human health; see, for
example, Section 7.5.2), a formal human health risk assessment need not be completed and documented before
taking a response action, but a preliminary evaluation of human health risk using individual building data or
aggregated community data is generally recommended (also see Section 7.8).
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The human health risk posed to building occupants by intrusion of a given vapor-forming
chemical will depend upon its toxicity, its concentration in indoor air, the amount of time the
occupants spend in the building, and othervariables (e.g., human life stage (e.g., child) can
matter for some chemicals (e.g., those with a mutagenic mode of action for carcinogenicity)).
EPA recommends that its risk assessment guidance (e.g., EPA 2009c, EPA 2003) be used to
identify, develop, and combine information about these variables and characterize human health
risk due to vapor intrusion from subsurface contaminant sources.
For the vapor intrusion pathway, the inhalation route is the primary means of human exposure.
Therefore, the human health risk assessment uses estimates of indoor air exposure
concentrations, exposure duration and frequency for building occupants, and the potential
toxicity of the vapor-forming chemicals found in the subsurface (e.g., inhalation unit risk and
noncancer reference concentration) to characterize risks of cancer and noncancer effects (EPA
2009c). Generally, exposure concentrations in existing buildings can be estimated using direct
measurements of indoor air (see Sections6.3.4 and 6.4.1). EPA recommends that time-
integrated measurements from multiple sampling events be used to estimate exposure
concentrations appropriate for the exposure (occupancy) scenario being evaluated (e.g.,
residential versus commercial), when the risk assessment for an existing building would support
a conclusion that the human health risk is acceptable (see Section 7.4.1).194'195 Generally,
modeling would be used to conservatively estimate exposure concentrations under future
conditions in buildings yet to be constructed in areas with subsurface contamination by vapor-
forming chemicals (see Section 6.6). EPA recommends the noncancer assessment consider the
potential for adverse health effects from short-duration inhalation exposures (i.e., acute, short-
term, or subchronic exposure durations),196 as well as longer term inhalation exposure (i.e.,
chronic exposure) conditions. EPA recommends that inhalation toxicity values be selected
194
An individual sample, collected at a randomly chosen time, may under-estimate (or over-estimate) average and
reasonable maximum exposure conditions. From their high-frequency, measured data, Holton et al. formulated a
synthetic data set (simulating one-day-average concentrations), which they used to estimate that a single, randomly
draw n, one-day sample had a forty percent chance of being less than the true mean (Holton etal. 2013b; see Table 1
therein). When the true mean was assumed to exceed the risk-based action level ("target concentration" in their
parlance) by two or five times, they estimated thatasingle, randomly drawn, one-day sample had a tw enty percent or
six percent chance, respectively, of not detecting the exceedance. These data support EPA's recommendation to
collect multiple rounds of indoor air sampling data to reduce the chance of reaching a false-negative conclusion (i.e.,
concluding exposure is at an acceptable risk level when it is not). Collecting multiple rounds of indoor air sampling
can also reduce the chance of reaching a false-positive conclusion (i.e., concluding that vapor intrusion poses
unacceptable human health risk when it does not), because an individual sample, collected at a randomly chosen
time, may over-estimate the average and reasonable maximum exposure conditions.
195
Given EPA's assigned mission to protect human health from environmental contamination and recognizing the
disruption to building owners and occupants caused by indoor air sampling, risk managers may choose to pursue
pre-emptive mitigation (i.e., early action) at some buildings (see Sections 3.3 and 7.8) rather than, for example,
conduct multiple rounds of sampling over a few years to establish an estimate of long-term average exposure
concentration and characterize temporal variability.
196
The inhalation reference concentration (RfC) (expressed in units of mass concentration in air) is defined as an
estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the
human population (including sensitive subgroups) that is likely to bewithoutan appreciable risk of deleterious effects
during a lifetime. Reference values may be derived for acute (<24 hours), short-term (>24 hours, up to 30 days),
subchronic (>30 days, up to approximately 10% of the life span), and chronic (greater than 10% of the life span)
exposure durations, all of which are derived based on an assumption of continuous exposure throughout the duration
specified. See http://www.epa.gov/ncea/iris/help ques.htm#w hatiris
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considering OSWER's hierarchy of sources (EPA 2003) and that relevant existing guidance
(e.g., EPA2009c) be followed in situations where a desired toxicity value is not available.
When a single vapor-forming chemical is present in the subsurface and intrudes as a vapor into
occupied building spaces, the noncancer human health risk can be characterized by calculating
the noncancer hazard quotient (HQ) (EPA2009c,Chapters). When multiple vapor-forming
chemicals are present in the subsurface and intrude as vapors into occupied building spaces,
the HQ estimates for each chemical are aggregated (as a simple sum, which is the Hazard
Index (HI)), based upon the assumption that each chemical acts independently (i.e., there are
no synergistic or antagonistic toxicity interactions among the chemicals). If the HI exceeds one,
there may be concern for potential adverse non-cancer effects and risk assessors should
consider segregating the chemicals by target organ or toxic effect to derive separate hazard
index (HI) values for each (EPA2009c, Chapters). EPA recommends that noncancer HQ and
HI values be estimated for each type of exposure period identified in the conceptual site model
or indicated by measurements of indoor air levels of vapor-forming chemicals (e.g., chronic,
subchronic, short-term, acute), evaluating inhalation reference concentrations that "match the
characterization of the exposure scenario" (EPA2009c, Chapter 4).197
The carcinogenic risks can be characterized by calculating the excess cancer risk over a lifetime
(LCR) and, if multiple vapor-forming chemicals are present, aggregating the LCR estimates for
each carcinogen (as a simple sum), based upon the assumption that each chemical acts
independently (EPA2009c, Chapters).
A well-crafted risk characterization section (EPA 1992c, 1995ab, 2000b, 2009c) puts risk
calculations into context for risk managers, so that they may effectively weigh and interpret risk
assessment results and recognize key uncertainties (e.g., in the exposure and dose-response
assessments and risk estimation).198 Additional recommendations for promoting and increasing
the utility and transparency of human health risk assessments can be found in Framework for
Human Health Risk Assessment to Inform Decision Making (EPA-RAF, 2014).
197
For example, when evaluating situations in which vapor concentrations in indoor air exceed the chronic reference
concentration (see Section 7.4.1), and there are shorter periods of significantly higher vapor intrusion exposure, EPA
recommends that noncancer risks for the shorter periods also be characterized using toxicity values appropriate for
the respective period(s). On the other hand, if vapor concentrations in indoor air are consistently less than
benchmarks for acceptable chronic exposure, then exposures forless-than-chronic scenarios are unlikely to pose
unacceptable human health risk
For example, EPA recommends that the risk characterization for existing buildings describe the uncertainty in the
exposure assessment arising from: (i) inherent variability of indoor air exposures overtime and space; (ii) the match
between the sampling data [e.g., sampling frequency (i.e., number of samples and time intervals between samples);
and time period over w hich each sample was collected] and the exposure period represented by the selected toxicity
value (e.g., chronic); and (iii) the ability to distinguish and apportion the contribution to indoor air concentrations
arising from vapor intrusion versus background sources. EPA recommends that the risk characterization for future
buildings describe the principal uncertainties in the exposure assessment, which may be associated with the type(s)
of building use, building construction and operations (e.g., HVAC system), frequency and duration of occupancy,
vapor concentrations in indoor air, or other factors.
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EPA recommends that any human health risk assessment be documented; a summary in a
decision document is also generally warranted.199 EPA also recommends that human health risk
information for individual buildings be communicated to building occupant(s) and owners.
Section 9.4 provides additional information for communicating sampling results.
7.4.1 Risk Management Benchmarks
EPA recommends that OSWER programs make the risk management determination to take
response action consistent with their statutes and regulations and considering existing program
guidance. 20ฐ The carcinogenic risk and non-cancer HI values used in this determination
generally are the "cumulative risks" that include all exposure pathways that a given population
may consistently face.201 In making such risk management determinations, EPA generally
recommends reporting the HQ and HI to one significant figure.
EPA generally uses a cancer risk range of 10"6 to 10"4 as a "target range" within which to
manage human health risk as part of site cleanup. For judging whether indoor air exposures
may pose acceptable health risk based upon potential non-cancer effects, EPA generally
recommends that the target HQ or HI not exceed 1.
Once a decision has been made to undertake a response action, EPA has expressed a
preference for cleanups that are at the more protective end of the cancer risk range. Thus, EPA
recommends using an individual lifetime cancer risk of 10"6 as a point of departure for
establishing cleanup levels based upon potential cancer effects (see Section 7.6).202 The EPA
risk manager may determine that a response action achieving reductions in human health risk
1QQ
Devices that have been found to improve comprehension and retention of textual materials include a table of
contents, clear section headings, and a summary (Morgan etal. 1992). It is most helpful to provide a summary that
translates the risk assessment into relatively simple language that non-expert risk managers, stakeholders, and wider
audiences can understand (Lundgren and Me Ma kin, 2013). Hazards and risks posed by vapor intrusion are more
likely to be misunderstood or misinterpreted if they are not explained in simple terms.
See, for example: The Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions, OSWER
Directive 9355.0-30, April 22, 1991 (EPA 1991 a); Rules of Thumb for Superfund Remedy Selection, OSWER
Drective 9355.0-69, August 1997 (EPA 1997); and Advanced Notice of Proposed Rulemaking: Corrective Action for
Solid Waste Management Units at Hazardous Waste Management Facilities (61 Federal Register 19432, May 1,
1996). So, for example, EPA cited OSWER Directive 9355.0-30 (EPA 1991 a) in its Compilation of Information
Relating to Early/Interim Actions at Superfund Sites and the TCE IRIS Assessment (EPA 2014b).
201 In some site-specific situations, a population might be exposed to a substance or combination of substances
through several exposure pathways (i.e., not only the vapor intrusion pathway). For example, individuals might be
exposed to substance(s) from a contaminated site by consuming contaminated drinking waterfroma groundwater
supply, as well as from vapor intrusion. Once reasonably expected exposure pathways have been identified, EPA
recommends examining whether it is likely that the same individuals would consistently face the reasonable
maximum exposure for each pathway or a combination of some of these pathways. Under such circumstances, the
total exposure to each chemical would equal the sum of the exposures by all consistently faced pathways (EPA 1989,
Section 8.3) and EPA recommends that the risk assessor clearly identify those exposure pathw ay combinations for
which a total risk estimate or hazard index is being developed. When characterizing human health risk arising from
multiple pathways and posed by a vapor-forming cherrical(s) with potential adverse noncancer effects, EPA
recommends that the toxicity values for each pathway "match the characterization of the exposure scenario" (EPA
2009c, Chapter 4).
202 See: National Oil and Hazardous Substances Pollution Contingency Ran (55 Federal Register 8717); and
Advanced Notice of Proposed Rulemaking: Corrective Action for Solid Waste Management Units at Hazardous
Waste Management Facilities (61 Federal Register 19432, May 1, 1996).
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within the 10"6 to 10"4 cancer risk range is acceptable, however, depending on site-specific
conditions or remedial factors.
For establishing cleanup levels based upon potential non-cancer effects, EPA generally
recommends that the target HI not exceed 1 (see Section 7.6).
7.4.2 Accounting for Background Contributions
As noted previously, EPA recommends including in the human health risk assessment vapor-
forming chemicals that are related to releases to the subsurface environment. Some of these
vapor-forming chemicals may be present in indoor air due to 'background' sources (see Section
2.7). If data are available, EPA recommends that the contribution of 'background' to total
exposure concentration(s) be distinguished in the human health risk assessment (EPA 2002e).
If background vapor sources (see Glossary) are found to be primarily responsible for indoor air
concentrations (see Section 6.3.5), then response actions for vapor intrusion would generally
not be warranted for current conditions. In any event, EPA recommends that the risk
characterization include a discussion of'background' contributions to indoor air exposure and
associated human health risk. With such information, EPA can help advise affected individuals
about the environmental and public health risks they face that are within their control (e.g.,
indoor sources of vapor-forming chemicals in residences).203 Other parties, including building
owners and operators, may help with risk communication.
If 'background' contributions are unknown and such data are sought to support risk
management decisions, EPA recommends that additional data be collected (see, for example,
Section 6.3.5). Information on 'background' contributions of site-related, vapor-forming
chemicals in indoor air is also important to risk managers because generally EPA does not
clean up to concentrations below natural or anthropogenic background levels (EPA2002e).
7.4.3 Occupational Exposure Limits
Permissible exposure limits (PELs) are enforceable occupational exposure standards developed
by the Occupational Safety and Health Administration (OSHA) in the U.S. Department of Labor.
Most of OSHA's PELs were adopted in 1971 from then-existing secondary guidance levels,
such as Threshold Limit Values (TLVs) developed by the American Conference of
Governmental Industrial Hygienists (ACGIH) to protect workers from adverse effects of
occupational exposure to airborne chemicals. They were intended to protect workers against
catastrophic effects (such as cardiovascular, liver, kidney, and lung damage), as well as more
subtle effects (such as narcosis, central liver system damage, and sensory irritation).
PELs (and TLVs), however, are not intended to protect sensitive workers, may not incorporate
the most recent toxicological data, and may differ from EPA derivations of toxicity values with
respect to weight-of-evidence considerations and use of uncertainty factors. For these and other
203 In cases where'background' contamination (e.g., due to indoor use of a consumer product or household chemical
in a residence) may pose a human health risk, but its remediation is beyond the authority of the applicable statute,
risk communication to the public may be most effective when coordinated with public health agencies (EPA 2002e).
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reasons,204 EPA does not recommend using OSHA's PELs (or TLVs) for purposes of assessing
human health risk posed to workers (EPA 1991 c, Appendix C) by the vapor intrusion pathway or
supporting final "no-further-action" determinations for vapor intrusion arising in nonresidential
buildings. Rather, EPA'srecommendationsfor assessing human health risk posed by vapor
intrusion are set forth herein in Sections 7.4.1 and 7.4.2.
7.5 Concentration Levels Indicating Potential Need for Prompt Response Action
In some circumstances, human health risk arises from vapor intrusion, which warrants prompt
response action. This Section provides some recommendations for identifying such
circumstances.
7.5.1 Potential Explosion Hazards
EPA recommends using the chemical-specific LELs205 to identify potential explosion hazards
(e.g., for methane and other petroleum hydrocarbons). Whenever building-specific data (such
as results from sub-slab soil gas samples and crawl space samples for any building type, indoor
air samples from sheds or pump houses, or gas samples from confined or semi-confined
spaces (e.g., sewers)) exceed one-tenth (10%) of the LEL for any chemical, a hazard is
indicated that generally warrants prompt action.206'207 EPA recommends to building owners and
occupants the evacuation of buildings with potential explosion and fire hazards, along with
immediate notification to the local fire department about the threat. Construction and operation
of engineered systems that can reduce or eliminate intrusion of explosive vapors into existing
buildings or unoccupied structures may also warrant consideration to reduce the potential for
future explosion hazards.
7.5.2 Considering Short-term and Acute Exposures
EPA may identify health-protective concentration levels forvapor-forming chemicals based upon
potential noncancer health effects that can be posed by air exposures over short-term or acute
exposure durations, considering EPA guidance for human health risk assessment (e.g., EPA
OSHA's website (May 2015) currently states: "OSHA recognizes that many of its permissible exposure limits
(PELs) are outdated and inadequate for ensuring protection of worker health. Most of OSHA's PELs wereissued
shortly after adoption of the Occupational Safety and Health (OSH) Act in 1970, and have not been updated since
that time. Since 1970, OSHA promulgated ... new PELs for 16 agents, and standards without PELs for 13
carcinogens. Industrial experience, new developments in technology, and scientific data clearly indicate that in many
instances these adopted limits are [also] not sufficiently protective of worker health. This has been demonstrated by
the reduction in allowable exposure limits recommended by many technical, professional, industrial, and government
organizations, both inside and outside the United States." [On-line source: https://www.osha.gov/dsa/annotated-pels/1
On October 10, 2014, OSHA issued a Chemical Management Request for Information (79 FR 61384), in which it
acknowledges many of its PELs are not sufficiently protective and seeks comment on strategies to address this
problem; available on-line at: https://www.osha.gov/FedReg osha pdf/FED20141010. pdf
205The Vapor Intrusion Screening Level Calculator (EPA 2015a) provides LELs forvapor-forming chemicals to
facilitate identification of potential explosion hazards.
NIOSH has designated such concentrations as immediately dangerous to life or health (IDLH).
207 Although the building-specific data may vary temporally, any short-term exceedance of one-tenth of the LEL
indicates vapor concentrations that, given an ignition source and available oxygen, may be capable of causing an
explosion.
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2009c) and using sources of toxicity information considering OSWER's hierarchy (EPA 2003).
For example, subchronic reference concentrations, developed by the EPA Office of Research
and Development/National Centerfor Environmental Assessment/Superfund Health Risk
Technical Support Center (STSC), are currently available for some vapor-forming chemicals as
Provisional Peer Reviewed Toxicity Values (PPRTVs), which are designated as a Tier 2 source
of toxicity values by OSWER (EPA 2003). Acute and intermediate Minimal Risk Levels
(MRLs)208 adopted by the Agency for Toxic Substances and Disease Registry (ATSDR) are
currently available for some vapor-forming chemicals and are designated as a Tier 3 source of
toxicity values by OSWER (EPA 2003). PPRTVs and ATSDR MRLs are peer reviewed and are
publicly available (see, http://hhpprtv.oml.gov/and http://www.atsdr.cdc.gov/mrls.html).
Historically, toxicity values for short-term or acute exposure durations have not been derived or
published in EPAs IRIS, which otherwise is EPAs preferred source of toxicity values (EPA
2003). EPA, under its authority, will work to develop expanded science policy direction to
address short-term exposures and develop and identify appropriate toxicity values for additional
chemicals for consideration forvapor intrusion assessment and related OSWER regulatory
frameworks. EPA recommends that relevant existing guidance (e.g., EPA2009c) be followed in
situations where a desired toxicity value is not available, using sources of toxicity information
considering OSWER's hierarchy (EPA 2003).
Although the indoor air concentrations may vary temporally, an appropriate exposure
concentration estimate (e.g..time-integrated or time-averaged indoor air concentration
measurement in an occupied space - see Section 6.4.1) that exceeds the health-protective
concentration levels foracute or short-term exposure (i.e., generally considered to be a hazard
quotient (HQ) greater than one for an acute or short-term exposure period)209 indicates vapor
concentrations that are generally considered to pose an unacceptable human health risk.210
As noted in Section 7.4 of this Technical Guide, a well-crafted risk characterization section (EPA
1992c, 1995ab, 2000b, 2009c) puts risk calculations into context for risk managers, so that they
may effectively weigh and interpret risk assessment results and recognize key uncertainties
(e.g., in the exposure and dose-response assessments and risk estimation). Uncertainties
include the derivation of an RfC, which is defined as "...an estimate (with uncertainty spanning
perhaps an order of magnitude)..." (See http://www.epa.gov/ncea/iris/help ques.htm#whatiris).
Sections 3.3 and 7.7 identify other EPA-recommended considerations for risk managers.
When indoor air concentrations in an occupied space exceed health-protective concentration
levels for short-term or acute inhalation exposures arising from a complete vapor intrusion
208 Minimal Rsk Levels (MRLs) published by ATSDR are estimates of the daily human exposure to a hazardous
substance that is likely to be without appreciable risk of adverse non-cancer health effects over a specified duration of
exposure. The ATSDR MRLs are peer reviewed and are publicly available (http://www.atsdr.cdc.aov/mrls.html).
See Glossary for definitions of "acute" and "short-term" exposure durations.
210 See, for example: The Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions, OSWER
Directive 9355.0-30, April 22, 1991 (EPA 1991 a); and Rules of Thumb for Superfund Remedy Selection, OSWER
Drective 9355.0-69, August 1997 (EPA 1997). In addition, the NCP states "For systemic toxicants, acceptable
exposure levels shall represent concentration levels towhichthe human population, including sensitive subgroups,
may be exposed without adverse effect during a lifetime or part of a lifetime, incorporating an adequate margin of
safety" [40 CFR 300.430(e)(2)(i)(1)].
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pathway, ventilation, indoor air treatment, temporary relocation, and other response actions may
be implemented to reduce or avoid these threats promptly (see Section 8.2.1). Construction and
operation of engineered systems that can reduce or eliminate vapor intrusion into existing
buildings (see Section 8.2) may also warrant consideration after urgent threats to human health
have been addressed.
7.6 Risk-based Cleanup Levels
When response action is determined to be warranted to reduce or eliminate indoor air
exposures from vapor intrusion (see Sections 7.4 and 7.5), EPA recommends that cleanup
levels be established and documented consistent with statutes and regulations and considering
guidance for the respective OSWER program.211 These cleanup levels would be used to
evaluate when building mitigation measures, subsurface remediation, and associated
monitoring can be terminated and to assess cleanup progress in the meantime (see Section
8.7).
Candidate risk-based cleanup levels can be calculated using information from the risk
assessment (Section 7.4). Results of the human health risk assessment indicate, for example,
which site-related vapor-forming chemicals warrant building mitigation and subsurface
remediation. The exposure factors and toxicity values used in the human health risk
assessment can be used to calculate chemical-specific cleanup levels, considering EPA risk
assessment methods (e.g., EPA2009c, EPA2003). Candidate cleanup levels are usually
developed for potential cancerand non-cancer effects. The lower (or lowest if there are multiple
potential non-cancer effects) of the candidate values, based upon cancer risk and non-cancer
HQ/HI targets, is generally recommended for selection as the cleanup level (EPA 1991 c,
Section 3.4 therein).212TheVISL Calculator
(http://www.epa.aov/oswer/vaporintrusion/auidance.htmn can be used to support these
calculations, including input of alternative attenuation factor(s) based upon site- or building-
specific information.
Calculating candidate cleanup levels based upon potential cancer effects entails selecting a
target cancer risk. As noted above (Section 7.4.1), once a decision has been made to undertake
a response action, EPA has expressed a preference for cleanups achieving the lower end of the
cancer risk range (i.e., 10"6) (EPA 1991 a). Response actions achieving reductions in human
health risk anywhere within the cancer risk range may be deemed acceptable by the EPA risk
manager, however.
To protect human health from potential noncancer effects, EPA generally recommends using a
target value of one for the non-cancer HQ (if there is a single vapor-forming chemical of health
concern for vapor intrusion) or for the non-cancer HI (if there are multiple vapor-forming
chemicals of health concern for vapor intrusion acting by a common effect).
211 See, for example: RCRA Corrective Action Plan (Final), OSWER Directive 9902.3-2A (EPA 1994); and A Guide to
Preparing Superfund Proposed Plans, Records of Decision, and Other Remedy Selection Decision Documents,
OSWER Directive 9200.1-23P (EPA 1999b).
919
An exception arises when'background' sources pose elevated exposures, because generally EPA does not clean
up to concentrations below natural or anthropogenic background levels (EPA 2002e).
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Cleanup levels for indoor air can be readily calculated, as described above, without additional
assumptions or modeling about vapor intrusion processes. On the other hand, cleanup levels for
groundwater and/orsoil gas in the vadose zone will entail developing a medium-specificvapor
attenuation factor, which EPA recommends be conservatively estimated based upon site-
specific information. The cleanup level for soil gas can be calculated by dividing the chemical-
specific indoor air cleanup level by the site-specific soil gas vapor attenuation factor. The
cleanup level for groundwater can be calculated by dividing the chemical-specific indoor air
cleanup level by the site-specific vaporattenuationfactorfor groundwater vapors and assuming
equilibrium between the aqueous and vapor phases at the groundwatertable. EPA
recommends that site-specific attenuation factors intended to be protective of chronic exposure
conditions: be conservatively estimated when based upon mathematical models; and be based
upon multiple measurements of indoor air concentration in different seasons, which have
negligible influences from'background' sources, when based upon site-specific measurements.
EPA recommends that cleanup levels be documented with at most two significant figures, even
though some of the input values may carry additional significant figures (EPA 1991 b, see page
19).
7.7 Options for Response Action
When response action is determined to be warranted to reduce or eliminate indoor air
exposures from vapor intrusion (see Sections 7.4 and 7.5), EPA recommends that OSWER
programs select, recommend, and document response action(s) consistentwith statutes and
regulations and considering their existing program guidance.213
The selection of a health-protective interim response action(s) for existing buildings will
generally depend on site-spedfic considerations, which can include: nature of subsurface vapor
source (e.g., groundwater, vadose zone soils, sewer lines), magnitude of the exposure above
cleanup levels; the severity of the potential adverse health effects or health hazard; building
features and conditions (e.g., construction; heating, ventilation, and air conditioning equipment);
climate and season (which influence the feasibility of ventilation, for example); the quality of
ambient air in the vicinity; and the feasibility of implementing a given option quickly. 14 In
general, EPA recommends that response actions limit the amount of time individuals are
exposed to concentrations that correspond to unacceptable human health risk, as described in
Sections 7.4 and 7.5.
213 See, for example: RCRA Corrective Action Plan (Final), OSWER Directive 9902.3-2A (EPA 1994); and A Guide to
Preparing Superfund Proposed Plans, Records of Decision, and Other Remedy Selection Decision Documents,
OSWER Directive 9200.1-23P (EPA 1999b).
Most response actions cannot be implemented immediately upon determining that a response is warranted. For
example, engineered exposure controls ordinarily entail from two to four weeks of lead time (at a minimum) for
planning, design, any permit acquisition, material acquisition and construction. In many circumstances, ventilation
measures to reduce exposure can be implemented more quickly, but local climate or air quality may render this
option less attractive during some seasons.
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TABLE 7-1
MATRIX OF OPTIONS TO RESPOND TO HUMAN HEALTH RISK
POSED BY THE VAPOR INTRUSION PATHWAY
Option for Response Action
Re mediation of Source*
Removal of contaminated soil via excavation
Treatment of contaminated soil in situ
Treatment of contaminated groundwater /ns/fu
Removal of contaminated groundwater(e.g., pump-and-treat)
Decontaminating and/or rehabilitating sewer and drain lines
Interim Measures to Reduce or Eliminate Vapor Intrusion*
Subslab de-pressurization and ventilation systems
Sealing major openings for soil gas entry, where known and
identified"1"
Building over-pressurization
Installing, repairing, or maintaining vapor traps
Interim Measures to Reduce or Avoid Exposure to Vapors
Notification to local fire department about potential explosion
hazards"1"
Notification and risk communication to building occupants and
owners, including institutional controls (e.g., deed notices)
Increasing building ventilation*
Treating indoor air*
Temporary relocation"1"
Monitoring Indoor Air to Characterize Human Exposure
Applicability of Response Action for
Common Sources of Sub-surface Vapors
Groundwater
#
#
Vadose
Zone Soil
Sewer &
Drain Lines
KEY: "designates potentially appropriate response action for indicated vapor source
FOOTNOTES:
* includes: associated institutional controls to maintain operations and provide public notification of residual
contamination; and associated monitoring to assess effectiveness and protectiveness of the response action
# remediation of soil may also be warranted for purposes of protecting groundwater from further contamination,
even if contaminated soil in the vadosezone is not a source for vapor intrusion directly (e.g., due to the absence
of an existing building near the contaminated soil)
+ response option primarily applies to existing buildings
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Table 7-1 presents an overview of key candidate response options for the vapor intrusion
pathway, which are discussed further in Section 8. Response actions that may be
recommended for and implemented in existing buildings include:
Interim measures that can be implemented relatively quickly (see Section 8.2.1), if
prompt action is warranted to reduce or eliminate exposures to vapor-forming chemicals
(see Sections 5.2 and 7.5.2) or to mitigate explosion hazards (see Section 7.5.1);
Engineered exposure controls (see Section 8.2.2) with associated monitoring and
institutional controls (see Section 8.6), as an interim (but potentially long-term) measure
to reduce or eliminate vapor intrusion into buildings; and
Remediation of the subsurface vapor source (see Section 8.1) with associated
monitoring and institutional controls (see Section 8.6).
Response actions that may be warranted in buildings that may be constructed in the future
include:
Remediation of the subsurface vapor source (see Section 8.1) with associated
monitoring and institutional controls (see Section 8.6); and
Institutional controls (see Section 8.6) to inform the need for building mitigation (see
Section 8.2.2) and/or a confirmatory vapor intrusion investigation before the building is
occupied, in case the building is to be or may be constructed before subsurface vapor
sources are remediated to cleanup levels.
Indoor air monitoring has frequently been selected as a response action in circumstances where
subsurface vapor sources are present and the vapor intrusion pathway has not been shown to
be incomplete. Indoor air monitoring may be deemed warranted, for example:
To better characterize spatial or temporal variability;
To address uncertainty in the characterization of the vapor intrusion pathway when
subsurface vapor sources have the potential to pose a health concern in overlying or
nearby buildings (e.g., incomplete pathway characterization, concern about the potential
for changes in building conditions, discordant lines of evidence); or
For other site-specific or situation-specific reasons.
EPA generally prefers to obtain building access and undertake response actions through
consent and cooperation from building owners, tenants, and other stakeholders (see Section
1.2).
7.8 Pre-emptive Mitigation/Early Action
It may be appropriate to implement mitigation of the vapor intrusion pathway as an early action,
even though all pertinent lines of evidence have not yet been completely developed to
characterize the vapor intrusion pathway for the subject building(s), when sufficient site-specific
data indicate that vapor intrusion: (1) is occurring or may occur due to subsurface contamination
that is being addressed by federal statutes, regulations, or guidance for environmental
protection; and (2) is posing or may pose a health concern to occupants of an existing
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building(s). Likewise, it may be appropriate and cost-effective to design, install, operate, and
monitor mitigation systems (including passive barrier systems) in newly constructed buildings
(or buildings planned for future construction) that are located in areas of vapor-forming
subsurface contamination, ratherthan allow vapor intrusion (if any) to occur and address vapor
intrusion after the fact. As described in Section 3.3, preemptive mitigation/early action is the
term used to describe both situations.
Preemptive mitigation (PEM) is recognized as an early action that is intended to ensure
protectiveness of human health. In this context and as described further in Section 8.2,
mitigation refers to methods that seek to:
Prevent or reduce vapor entry into a building.
Reduce or eliminate vapors that have entered a building.
Note that the selection and implementation of PEM, when it occurs, is not necessarily intended
to pre-judge final decisions about remediation of subsurface vapor sources; however, EPA
generally recommends that decision-making about PEM include a consideration of the O&M
and monitoring obligations. In addition, EPA recommends that the selection of PEM be based
upon data and information in the administrative record and be documented in the administrative
record, consistent with statutes and regulations and considering EPA guidance for the
respective land restoration program (e.g., CERCLA, RCRA corrective action, brownfields, etc.),
in order to provide an adequate basis for actions undertaken.
7.8.1 Rationale
In ensuring protectiveness of human health, PEM generally may be an appropriate approach to
consider for buildings with potential vapor intrusion for a number of reasons, including:
Building mitigation typically is an effective means of protecting human health and is
cost effective for many buildings.
The potential exposure scenario (e.g., inhalation of potentially toxic vapors) cannot
generally be readily avoided by building occupants.
Involuntary and unavoidable exposures and hazards are generally sources of anxiety
and concern for affected building occupants and the general public, particularly when
they occur in homes and in the workplace.
Comprehensive subsurface characterization and investigations of vapor intrusion (to
conclusively characterize unacceptable, but variable, levels of vapor-forming
chemicals in soil, groundwater, and indoor air, as described in Section 6) can entail
prolonged study periods, during which building occupants may be exposed and
owners and environmental stewardship groups may remain anxious and concerned
about potential indoor air exposures to subsurface vapors in the absence of
mitigation.
Conventional vapor intrusion investigations in and of themselves can be disruptive,
particularly when indoor access is sought to acquire interior samples and assess
interior building conditions.
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Mitigation can typically be implemented relatively quickly, while subsurface
contamination is being more fully delineated or remediated.
EPA's experience with residential communities suggests that many affected
residents seek and preferthat mitigation systems be installed when vapor intrusion is
suspected.
Mitigation can be a cost-effective approach to help ensure protectiveness of human
health during ongoing vaporintrusion investigations to acquire multiple lines of
evidence and characterize spatial and temporal variability in subsurface and indoor
air concentrations, as well as while subsurface remediation is being planned and
conducted to reduce or eliminate subsurface vapor sources.
In summary, PEM, based on limited but credible subsurface and building data, can be an
appropriate approach to begin to implement response actions quickly and ensure protectiveness
of current building occupants. In such circumstances, resources can be used appropriately to
focus first on mitigation of buildings and subsurface remediation, rather than site and building
characterization efforts, which may be prolonged. Although PEM may be an effective tool to
reduce the human exposure and human health risk, building mitigation is not generally intended
to address the subsurface vaporsource; as such, EPA recommends that it typically be used in
conjunction with remediation of the subsurface source of vapor-forming chemicals (e.g., source
removal or treatment), as discussed in Section 8.1.
7.8.2 General Decision Framework
To consider PEM, EPA recommends that reliable data supporting a preliminary analysis, as
described in Section 5.0, and risk-based screening, as described in Section 6.5, be obtained
and documented in the administrative record. In appropriate circumstances (e.g., where time is
of the essence to ensure protection of human health; see, for example, Section 7.5.2), a formal
human health risk assessment need not be conducted and documented before selecting PEM,
but a preliminary evaluation of human health risk using individual building data or aggregated
community data is generally recommended. If there are insufficient data to perform a preliminary
risk analysis, but subsurface vapor sources are known to be present near buildings (see Section
5.3), EPA recommends that an appropriate vapor intrusion investigation (see Section 6) be
conducted to obtain sufficient data.
EPA generally recommends that the decision to undertake building mitigation be supported by
appropriate lines of site- or building-specificevidence (e.g., characterization of subsurface vapor
source(s) strength and proximity to building(s); building conditions) that demonstrate that vapor
intrusion has the potential to pose an unacceptable human health risk. Sections 5, 6, and 7
herein provide information about the types of evidence obtained and relied upon in assessing
vapor intrusion potential and the types of analyses that can support determinations of whether
the vapor intrusion pathway is complete for a specific building or collection of buildings and
poses or has the potential to pose a health concern to building occupants. This information is
equally pertinent for supporting final remediation and mitigation decisions and for supporting
PEM consistent with applicable statutes and regulations. The premise of PEM, however, is to
protect human health first without necessarily waiting to collect all lines of pertinent evidence or
multiple rounds of sampling data.
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Certain types of subsurface conditions may have greater potential to facilitate vapor intrusion
when subsurface sources of vapors are present. These conditions include, but are not limited to:
Shallow aquifers (for example, five feet or less from the building foundation to the
seasonal high water table).
High-permeability (e.g., gravelly) vadose zone soils that are fairly dry, which are
favorable to upward migration of gases.
Preferential migration routes, such as fractured sediments or bedrock, buried
streambeds, subsurface drains, and utility conduits, as they can facilitate vertical or
lateral migration of vapor with limited attenuation of chemical concentrations.
Under these conditions, it may be easier to determine that PEM may be warranted if a structure
is located near a subsurface vapor source that has the potential to pose an unacceptable
human health risk. Other factors to consider include the following:
Susceptibility to soil gas entry. Some buildings have greater potential for vapor intrusion
(i.e., are more susceptible to soil gas entry; see Section 2.3) than others. For example,
buildings with deteriorating basements or dirt floors generally provide poor barriers to
vapor (soil gas) entry. Buildings with sumps or other openings to the subsurface that can
facilitate soil gas entry are also more susceptible to vapor intrusion.
Actions undertaken or planned to address the subsurface source of vapors. For
example, if the source of vapors (e.g., contaminated soil in the vadose zone) is being
removed (e.g., excavation of contaminated soil or soil vapor extraction underneath the
building) or is to be removed within a time frame that is protective for any potential
current or near-term exposures in the overlying or nearby building, then PEM may not be
warranted.
7.8.3 Some General Scenarios Where Pre-emptive Mitigation May be Warranted
Three general scenarios where PEM may be warranted are summarized below. The first two
scenarios address situations where building(s) currently exist, while the third scenario
addresses a situation where building(s) may be constructed in the future.
Site with High Potential to Facilitate Vapor Intrusion. In this scenario, indoor air concentration
data have not been collected, but other lines of evidence support a conclusion that the vapor
intrusion pathway is likely complete and may pose an unacceptable human health risk. Figure 7-
1 shows a hypothetical residential area located near a shopping center that contains an active
dry-cleaning facility. In this hypothetical example, a sufficient number of appropriately screened
monitoring wells have been installed throughoutthe neighborhood to characterize a historical
groundwater plume emanating from the dry cleaner that has migrated under eight homes and
continues to migrate. Groundwater is encountered at approximately five feet below ground
surface, and site geology consists of dry gravel and sands. "Near-source" soil gas samples have
also been collected from several locations throughout the neighborhood and found to
corroborate a high-strength vapor source near the buildings. All homes have crawl spaces with
dirt floors. In this hypothetical example, PEM may be warranted for the eight buildings located
above, near, or downgradient of the groundwater plume, based on the groundwater
concentration and soil gas data available (i.e., PCE concentrations significantly exceeding
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screening levels in this example), 215and the likelihood thatthe proximity of the groundwater
table, characteristics of the vadose zone, and building conditions will collectively facilitate vapor
migration and intrusion.
Note that if a groundwater restoration system is constructed and operated and plume migration
is thereby controlled, additional buildings downgradient of the plume may not warrant PEM in
the future. In the meantime, an 1C may be appropriate for the undeveloped parcel hydraulically
down-gradient of the current leading edge of the plume.
Site with Indoor Air Data for Some Buildings but Not for All Buildings. Depending on individual
owners and occupants in the affected community, it may be difficult to obtain adequate data for
all buildings within a specified area. Challenges include gaining timely access into each building
and other practical considerations. In such circumstances, it may be appropriate to characterize
a limited number of buildings under a reasonable maximum vapor intrusion condition,216 by
collecting and weighing multiple lines of evidence, and then extrapolating those findings to
similar buildings nearby. The following hypothetical scenario describes one such situation,
which is represented in Figure 7-2. In this hypothetical example, a sufficient number of
appropriately screened monitoring wells have been installed throughout the neighborhood to
characterize a historical groundwater plume. "Near-source" soil gas samples have also been
collected from several locations throughoutthe neighborhood and found to corroborate the
measured groundwater concentrations. Indoor air has been sampled and analyzed for a few
homes and found to exhibit concentrations that pose an unacceptable human health risk. In this
scenario, the assumption can be made that buildings with similar construction and built about
the same time may have similar susceptibility to soil gas entry. As a result, it may be determined
to use a PEM approach to offer mitigation systems to all buildings within a specified area of
subsurface contamination.
215
Several site-specific factors render inappropriate the use of the recommended attenuation factors and
groundwater and soil gas VISLs for purposes of identifying sites or buildings unlikely to pose a health concern
through the vapor intrusion pathway, as discussed in Section 6.5.2. Nevertheless, response actions forvapor
intrusion can be supported when the groundwater and soil gas VISLs are exceeded for samples from a building or
site where these specific factors are present.
216 EPA recommends basing decisions about whether to undertake response action forvapor intrusion (i.e., a
component of risk management) on a consideration of a reasonable maximum exposure (e.g., EPA 1989, 1991a),
which is intended to be a semi-quantitative phrase, referring to the lower portion of the high end of the exposure
distribution (see Glossary).
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A
GW Flow
A.
.
A
Park
I I
0 500 feet
No Vapor Intrusion
Pre-emptive Mitigation
Monitoring Well
GW VOC Plume
Figure 7-1 Sample Depiction of Subsurface Vapor Source and Data to Support
Pre-emptive Mitigation/Early Action for Multiple Buildings, Each with Limited Data
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A
6
A A
A
A A A A A
6 6
^66
*, 6 6 ฃ
A
A
A
A
A A
A
0 500 feet
GWVOC Plume
No Vapor Intrusion
Pre-emptive Mitigati'on
Confirmed Vapor Intrusion
Figure 7-2 Sample Depiction of Subsurface Vapor Source and Data to Support
Pre-emptive Mitigation/Early Action for Multiple Buildings, Some with Only Limited or No Data
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Future Construction and Development. If current data (e.g., "near-source" soil gas) indicate that
there is potential for unacceptable human health risk arising from vapor intrusion in an area
where a building(s) is expected to be constructed in the future, EPA recommends that the
remediation decision document record the known facts and data analyses and clearly state that
vapor intrusion mitigation or site re-evaluation may be needed when the property is developed
or occupied. EPA generally recommends appropriate ICs to ensure enforcement of such
remediation decisions.217
Prior site use (see Section 5) can be particularly relevant where residential development is
planned or occurring on property formerly used for commercial or industrial purposes. In these
situations, it is not uncommon for residual NAPLs or shallow plumes to remain. Under this
circumstance, PEM may be warranted for new construction as a precautionary measure without
direct evidence of a vapor intrusion pathway. Incorporating mitigation systems into newly
constructed buildings is generally easier to implement and incurs lower cost when compared
with retrofitting existing structures.
On the other hand, if response actions to treat or remove the subsurface vapor source(s) are
being conducted or will be conducted before a building is constructed and occupied, then
building mitigation for the vapor intrusion pathway may not be warranted in the future.
7.8.4 Additional Considerations
EPA recommends that the following factors also be considered in evaluating PEM and
determining whether to implement it.
Weighing Relative Costs of Characterization versus Engineered Exposure Controls. EPA
recommends that cost not be the primary criterion for deciding whether or howto mitigate vapor
intrusion because protection of human health could be compromised. On the other hand, cost
effectiveness is addressed by CERCLA and the NCP and can be an important consideration
when evaluating response alternatives. Cost can be a factor in deciding when and whether to
pursue PEM, in relation to continuing to investigate and assess actual or potential vapor
intrusion, and in ensuring effective human health protection through installing and operating a
vapor intrusion mitigation system. At PRP-lead sites, for example, PEM may be viewed
favorably where the costs associated with a complete site characterization or continued
monitoring are estimated to easily exceed the cost of installing a mitigation system (and
associated system monitoring). The number of buildings that would need to be characterized, or
the order of priority, may be a factor in considering whether to implement PEM.
Institutional Controls. For existing vapor intrusion mitigation systems, ICs may be warranted to
ensure that the system is operated, maintained, and monitored. Maintenance and monitoring of
the mitigation system, which are discussed in Sections 8.3 and 8.4 of this document, are
generally appropriate to ensure that the system is performing as intended. In addition, ICs may
facilitate access to property to conduct routine maintenance and monitoring activities, although
217 At undeveloped sites, or at sites where land use may change in the future, ICs may be important to ensure that
the vapor intrusion pathway is effectively addressed in the future. ICs at undeveloped sites could include mechanisms
to inform the need for PEM in new buildings. Selecting and implementing PEM for new buildings avoids some of the
difficulties associated with attempting to predict the potential for vapor intrusion prior to building construction.
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separate access agreements also warrant consideration. Additional information regarding ICs is
provided in Section 8.6 of this document.
Community Input and Preferences. Community acceptance of early action may vary widely,
depending on the human health risk to building occupants and past experiences at the site,
including interaction with site stakeholders and regulators and perceptions of the site. Some
owners and occupants may view PEM as a precautionary measure and be willing to have
mitigation systems installed; some may even request them before characterization is completed.
On the other hand, some home owners may not agree to have a mitigation system installed
unless the pathway is demonstrated to be complete.
Others may be reluctant to install mitigation systems because of the operation costs or the
inconvenience associated with the installation and subsequent monitoring. Although some
owners may view mitigation systems as an advantage when they sell a property, others may be
concerned with the possible negative effect on property values.
Issues and concerns about equity and fairness can also arise when some homes within a
neighborhood receive mitigation systems and others do not. In some situations, it may be easier
to persuade property owners to install vapor intrusion mitigation systems if the entire street,
block, or neighborhood is found to warrant early action.
Public meetings and one-on-one meetings provide opportunities to discuss PEM with affected
property owners and building occupants and obtain information and input. Section 9.0 of this
document provides additional information about community involvement and engagement.
Refined Conceptual Site Model
Decisions to undertake pre-emptive mitigation may warrant re-evaluation as additional
monitoring and/or site and/or building characterization data become available and are evaluated
in the context of the conceptual site model. If and when such data shift the weight of evidence
towards a conclusion that the vapor intrusion pathway is incomplete or otherwise does not pose
unacceptable health risk, then EPA recommends re-considering whether continued operation,
maintenance, and monitoring of the interim response action (Section 8.2) is warranted.
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8.0 BUILDING MITIGATION AND SUBSURFACE REMEDIATION
This section summarizes information and recommendations on potential response options to
mitigate and manage vapor intrusion. It is organized as follows:
Section 8.1 summarizes the role of subsurface remediation in mitigating vapor intrusion.
Section 8.2 provides an overview of engineered exposure controls (i.e., building
mitigation technologies) for existing and new buildings.
Sections 8.3 and 8.4 summarize information about operating and monitoring building
mitigation systems, respectively.
Section 8.5 summarizes information about documenting building mitigation systems.
Section 8.6 describes and provides information about institutional controls (ICs).
Section 8.7 provides information about exit strategies (e.g., termination of: subsurface
remediation for vapor source control; building mitigation system operation; and
associated ICs).
Sections 5.2, 7, and 9 discuss potential bases for deciding to implement response options for
vapor intrusion. Sections 3.3 and 7.7 introduced some of the response options and policies
discussed in the remainder of this Section regarding components and development of cleanup
plans.
8.1 Subsurface Re mediation for Vapor Source Control
The preferred long-term response to the intrusion of vapors into buildings is to eliminate or
substantially reduce the level of contamination in the subsurface vapor source (e.g.,
groundwater, subsurface soil, sewer lines) by vapor-forming chemicals to acceptable-risk levels,
thereby achieving a permanent remedy. Remediation of the groundwater plume or a source of
vapor-forming chemicals in the vadosezone will eventually eliminate potential exposure
pathways and can include the following actions, among others:
Removal of contaminated soil via excavation;
Removal of contaminated groundwaterwith pump-and-treat approaches;
Decontaminating and/or rehabilitating sewer lines that harbor vapor-forming chemicals;
and
Treatment of contaminated soil and groundwater in situ, using technologies such as soil
vapor extraction, multiphase extraction, and bioremediation, or natural attenuation.
Because there is a substantial body of EPA and other guidance on selection, design,
construction, and operation of technologies for remediation of subsurface vaporsources (e.g.,
EPA 1993b, 2006c; NRC 2004), these topics are not discussed furtherhere.
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When monitoring to assess the performance and effectiveness of remediation technologies for
subsurface vapors, EPA recommends employing the methods, approaches, and
recommendations described in Sections 5.4, 6.2, 6.4 and 7.0 of this Technical Guide.
ICs may be necessary to help ensure the continued integrity of the cleanup. In some cases,
therefore, ICs such as zoning or deed restrictions, may accompany implementation of vapor
source remediation methods. Section 8.6 provides information about ICs and their application to
the vapor intrusion pathway.
8.2 Building Mitigation for Vapor Intrusion
In cases where subsurface vapor sources cannot be remediated quickly, it may be appropriate
to also undertake (interim) measures in individual occupied buildings (i.e., building mitigation for
vapor intrusion) to reduce threats to human health more quickly. EPA recommends that building
mitigation for vapor intrusion be regarded as an interim action that can provide effective human
health protection, which may become part of a final cleanup plan. Mitigation of vapor intrusion in
specific buildings generally is not a substitute for remediation of subsurface vapor sources.
Thus, EPA recommends that building mitigation generally be conducted in conjunction with
vapor source remediation where at all possible.
The purpose of this section is to provide an overview of vapor intrusion mitigation for new and
existing buildings where response action is determined to be warranted. Section 8.2.1
summarizes response options that generally can be implemented relatively quickly to reduce
indoor air concentrations. Section 8.2.2 identifies and summarizes the most commonly
implemented engineered control methods for reducing vapor intrusion into existing buildings.
Section 8.2.3 identifies and describes some approaches and considerations for addressing
vapor intrusion for new buildings. Additional detailed information about technologies for redudng
vapor intrusion into buildings and their selection, design, operation, and monitoring is provided
in other EPA documents (EPA 1993a, 2008c). Building owners and occupants may find EPAs
Consumer's Guide to Radon Reduction (EPA 2013b) a useful source of additional information,
in light of similarities in technologies for reducing vapor intrusion and radon intrusion.
ICs may be necessary to help ensure the continued integrity of building mitigation systems. In
many cases, therefore, ICs may accompany implementation of engineered exposure controls,
for example to ensure that an active system remains operational and passive membranes are
not disturbed (EPA 2008c). Additional information about ICs is provided in Section 8.6.
8.2.1 Prompt Response Options for Existing Buildings
For buildings with potential explosion and fire hazards, EPA recommends evacuation, along
with notification of the local fire department about the threat. If, on the other hand, prompt action
is warranted to reduce or eliminate vapor intrusion exposures in an existing building (e.g.,
measured indoor air concentrations pose an unacceptable human health risk for an acute or
short-term exposure scenario (see Section 7.5.2)), it may be appropriate to implement response
options such as the following:
Sealing major openings for soil gas entry, where known and identified;
Over-pressurizing nonresidential buildings by adjusting the HVAC system;
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Installing, repairing, or maintaining vapor traps for sewer or drain lines that are sources
of vapor intrusion;
Increasing building ventilation, for example using fans or natural ventilation;
Treating indoor air (e.g., adsorption using activated carbon); and
Temporary relocation
The fore-going response actions may take several days to a fewweeks to plan (e.g., arrange,
design) and implement, which generally is quicker than other interim response actions (e.g.,
active depressurization technologies). The first three options seek to reduce or eliminate vapor
entry into the building. The last three options seekto reduce, eliminate, or avoid vapors that
have entered the building by vapor intrusion. Specifically:
Vapor intrusion into the building via soil gas entry from vadose zones soils can be
reduced by sealing foundational openings using products such as synthetic rubbers,
acrylics, oil-based sealants, asphalt/bituminous products, swelling cement, silicon, epoxy
or elastomeric polymers. EPA recommends screening the selected sealant(s) (e.g.,
checking the composition, relying upon manufacturer's data) to ensure they do not
contain or emit vapor-forming chemicals that might pose a human health risk to building
occupants. This interim mitigation approach is among the easiest and least expensive to
implement; however, its effectiveness relies upon being able to identify and access
openings for soil gas entry. EPA recommends appropriate monitoring of indoor air
concentrations be conducted to ensure that sealing attains and sustains sufficient
reduction in vapor intrusion. In some cases, however, sealing openings may not be
capable of reducing indoor air concentrations to acceptable levels and/or some openings
may not be visible and accessible. EPA recommends that this response option generally
be supplemented by installing, operating, and maintaining an engineered exposure
control (e.g., an active depressurization technology) that reduces or eliminates vapor
entry into the building until remediation of subsurface vapor sources is complete and
terminated.
For commercial and industrial buildings where HVAC units blow air into the building and
are well maintained, it may be advantageous to increase pressurization in the building to
prevent or reduce vapor intrusion. In some cases (e.g., buildings with few doors and
other openings), relatively small increases in building pressure may be sufficient, which
may be accomplished by increasing the air flow rate and using specialized equipment to
monitor and balance airflow rates. EPA recommends appropriate monitoring of pressure
and other indicators (e.g., indoor air monitoring) be conducted to ensure that adequate
pressurization is sustained throughout areas of the building that could be subject to
vapor intrusion. In some climates and for some buildings, this response option may be
impractical or prohibitively expensive.
Vapor intrusion into the building via gas entry from sewer and drain lines can be reduced
or eliminated by installing, repairing, and maintaining vaportraps.
Increasing building ventilation (i.e., increasingthe rate at which indoor air is replaced
with outdoor air) can reduce the buildup of indoor air contaminants within a structure.
Natural ventilation may be accomplished by opening windows, doors, and vents. Forced
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or mechanical ventilation may be accomplished by using a fan to blow air into or out of
the building. Increased ventilation is easiest and least costly to implement in locations
where the air is not conditioned (heated or cooled). If indoor air is conditioned, increased
ventilation can be a costly option because the conditioned air is ventilated to the
outdoors. This drawback can be partly overcome by use of heat exchangers, but they
are also costly. Another concern is that exhausting air from the building will generally
contribute to under-pressurization of the building, relative to the subsurface, thereby
potentially resulting in an increased rate of soil gas entry (i.e., vapor intrusion) unless
ambient air entry into the building is increased equivalently. EPA recommends
appropriate monitoring of indoor air concentrations be conducted to ensure that
ventilation attains and sustains sufficient reduction in exposures to vapor-forming
chemicals. In some cases, ventilation may not be capable of reducing indoor air
concentrations to acceptable levels. In addition, building occupants may find it
uncomfortable to increase the air exchange rate by more than a factor of three or four.
EPA generally recommends that this response option be supplanted, when feasible, by
installing, operating, and maintaining an engineered exposure control that reduces or
eliminates vapor entry into the building until remediation of subsurface vapor sources is
complete and terminated.
Commercially available indoor air cleaners, which include both in-duct models and
portable air cleaners, is another response option. These devices operate on various
principles, including zeolite and carbon sorption and photocatalytic oxidation. Methods
that rely on adsorption generate a waste that must be disposed of appropriately or
regenerated and warrant periodic replacement of the adsorption medium. EPA
recommends appropriate monitoring of indoorair concentrations be conducted to ensure
that adequate treatment is sustained throughout the building. EPA generally
recommends that this response option be supplanted, when feasible, by installing,
operating, and maintaining an engineered exposure control that reduces or eliminates
vapor entry into the building until remediation of subsurface vaporsources is complete
and terminated.
Temporary relocation may be implemented for buildings where conditions warranting
prompt response action (see Section 7.5) and cannot be adequately addressed by other
means.218
None of these options entails reducing the level of vapor-forming contamination in the
subsurface medium (see Section 8.1). EPA generally recommends that these response options
be supplanted, when feasible, by installing, operating, and maintaining an engineered exposure
218 For response actions carried out under Sections 104(a) and 106(a) of CERCLA, OSWER Directive 9230.0-97
(Superfund Response Actions: Temporary Relocations Implementation Guidance (EPA 2002d)) states: "Temporary
relocation should not be selected if health and safety risks or circumstances that pose an unreasonable
inconvenience can be adequately addressed by other means without significantly increasing the overall cost or
duration of the response action." Similarly, OSWER Directive 9355.0-71 P (Interim Policy on the Use of Permanent
Relocations as Part of Superfund Remedial Actions (EPA 1999f)) states: "EPA's preference is to address the risks
posed by the contamination by using well-designed methods of cleanup w hich allow people to remain safely in their
homes and business." OSWER Drective 9230.0-97 provides recommended procedures and other policies for
temporarily relocating residents when this response action is selected and implemented under Sections 104(a) and
106(a) of CERCLA.
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control that reduces or eliminates vapor entry into the building (see Section 8.2.2), until
remediation of subsurface vaporsources is complete and terminated.
8.2.2 Active Depressurization Technologies for Existing Buildings
This section provides a brief overview of engineered systems that can be used to reduce or
eliminate soil vapor intrusion in existing buildings, along with a summary of steps and
considerations for selecting an appropriate technology for a given building. The focus is on
active depressurization technologies most commonly employed for reducing soil vapor intrusion
into buildings. This focus does not mean, however, that active depressurization technologies are
always preferred over other mitigation methods or that they will be the best option for every site.
More detailed information on vaporintrusion mitigation systems for existing buildings, including
sub-membrane ventilation systems and passive technologies,219 can be found in several EPA
publications (e.g., EPA2008c).
Active depressurization technologies (ADT) have been used successfully to mitigate the
intrusion of radon into buildings and have also been successfully installed and operated in
residential, commercial, and school buildings to control vaporintrusion from subsurface vapor-
forming chemicals. ADT systems are widely considered the most practical vapor intrusion
mitigation strategy for most existing buildings, including those with basement slabs or slab-on-
grade foundations. ADT systems are generally recommended for consideration for vapor
intrusion mitigation because of their demonstrated capability to achieve significant concentration
reductions in a wide variety of buildings220 and their moderate cost.
Sub-slab depressurization (SSD) systems, a common type of ADT system, function by creating
a pressure difference across the building slab to prevent soil gas entry into the building (i.e.,
overcoming the building's natural under-pressurization, which is the 'driving force' for vapor
intrusion; see Section 2.3). Creating this pressure difference is accomplished by extracting soil
gas from beneath the slab and venting it to the atmosphere.221 Forthe system to be effective by
this mechanism,222 this soil depressurization must be established and maintained at least near
the primary openings for soil gas entry (EPA 1993a). Construction of SSD systems entails
opening one or more holes in the existing slab, removing soil from beneath the slab to create a
"suction pit" (6-18 inch radius), placing vertical suction pipes into the holes, and sealing the
219
As noted in Section 3.3, engineered exposure controls that do not involve mechanical operations (e.g., creating a
barrier between the soil and the building that blocks openings from soil gas entry into the building) are referred to as
"passive."
Folkes and Kurz (2002) describe a case study of a vapor intrusion mitigation program in Denver, Colorado. Sub-
slab depressurization systems and/or sub-membrane depressurization systems were installed in 337 residential
homes to control indoor air concentrations of 1,1-dichloroethylene (DCE) resulting from migration of vapors from
groundwater with elevated 1,1-DCE concentrations. Over three years of monitoring datafor301 homes have shown
that these systems are capable of achieving the very substantial reductions in concentrations in indoor air.
Approximately one quarter of the systems warranted minor adjustment or upgrading after initial installation in order to
achieve the state standards established for indoor air exposure.
221 Depending, in part, upon location and prevailing statutes and regulations, governmental permits or authorizations
may be required for venting systems that exhaust to the atmosphere.
222 A second mechanism by which ADT systems can function is diluting the vapor concentrations beneath the slab
and foundation (EPA 1993a, 2008c). Engineered controls designed and operated to use this mechanism
predominately are often referred to as sub-slab ventilation systems.
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openings around the pipes. These pipes are then connected togetherto a fan, which draws soil
gas from the sub-slab area through the piping and vents it to the outdoors.223 Sealing known
and accessible openings in the slab and foundation can reduce the flow rate of conditioned
indoor air that can be pulled into the sub-slab region by the suction wells and the sub-slab
depressurization (EPA2008).
SSD systems were first developed for radon reduction (EPA 1993a) and operate undersimilar
design principles as radon mitigation methods. Figure 8-1 illustrates such an SSD system.
When sumps and associated drain tile systems are present, they may also be depressurized to
prevent soil gas entry into the building (again, overcoming the building's natural under-
pressurization). This variation on active depressurization is often referred to as drain-tile
depressurization (DTD). Depressurization of drain tiles located neara foundation wall can help
control soil gas entry at the joint between the foundation wall and slab. Figure 8-2 illustrates
such a DTD system.
If the building has hollow block walls, the usual sub-slab suction point may not adequately
mitigate the wall cavities, which may be particularly important if the outside surfaces are in
contact with the soil. In these situations, the void network within the wall may be depressurized
by drawing airfrom inside the wall and venting it to the outside. This method, called "block-wall
depressurization" (BWD) is often used in combination with SSD. Because uniform
depressurization of block walls can be difficult, BWD is generally recommended only when sub-
slab or DTD prove inadequate to control vapor intrusion. Figure 8-3 illustrates such a BWD
system.
In buildings with a crawl space foundation or a basement with a dirt floor, a flexible membrane
may be installed over the floor to facilitate depressurization of the soil gas beneath the
membrane, which prevents vapors from intruding into the crawl space or basement air. To
maximize the effectiveness of a sub-membrane depressurization (SMD) system, EPA
recommends the membrane cover the entire floorarea and be sealed at all seams and
penetrations. Figure 8-4 illustrates such an SMD system.
Extensive guidance is available for the design, sizing, installation, and testing of ADT systems
for radon control in existing and new homes and large institutional (e.g., school) and commercial
buildings. EPA recommends that ADT systems be designed and installed by qualified persons,
typically environmental professionals and licensed radon contractors. EPA guidance fordesign
of ADT systems can be found in several publications(e.g., EPA1993a, 2008c). EPA
recommends documenting each constructed ADT system via a system manual, as described
further in Section 8.5.
The Vapor Intrusion Mitigation Quick Guide provided in Table 8-1 summarizes a list of steps for
selecting and implementing a vapor intrusion mitigation system in existing buildings.
223 A central issue that determines the design and effectiveness of ADT systems is the ease w ith w hich suction at one
location can extend to other subsurface areas underneath the building. Where a good and uniform layer of aggregate
(e.g., gravel or crushed rock) was placed underneath a slab foundation during construction, for example, such
hydraulic control and communication can generally be expected to be good (EPA 1993a). Where the layer of
aggregate under a slab is interrupted or uneven to a significant degree, additional suction pipes will generally be
needed and their location will be increasingly important.
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TABLE 8-1
VAPOR INTRUSION MITIGATION QUICK GUIDE FOR EXISTING BUILDINGS
Step 1: Consider Prompt Response Actions
It may be appropriate to implement certain interim measures before engineered controls are constructed and
operated, as warranted and feasible. For example, building ventilation can be increased, cracks and other openings
in the floor or foundation (that otherwise allow soil gas entry) can be sealed, or indoor air treatment can be conducted
(referto Section 8.2.1).
Step 2: Select a Building Mitigation System
The initial step in selecting the appropriate vapor intrusion mitigation technology is to conduct a visual inspection of
an existing building. The selection of a vapor intrusion mitigation system primarily depends on building characteristics
and contaminant concentrations. In the majority of cases, a type of active depressurization technology (ADT) can be
an efficient, reliable, and cost-effective vapor intrusion mitigation technique. In some cases, however, other
approaches may be preferable.
Factors that may prompt consideration of vapor intrusion mitigation approaches other than ADT include foundation
conditions that prevent development and extension of a suction field below the building.
If there are no factors that would rule out an ADT technology, appropriate systems that can be considered include:
Sub-slab depressurization (SSD) systems, particularly in houses having slabs (basements and slabs on grade)
where drain tiles are not present.
Drain-tile depressurization (sump/DTD or remote discharge/DTD) when drain tiles are present.
Sub-membrane depressurization (SMD) in buildings witha craw I space foundation or a basement witha dirt floor,
Block-wall depressurization (BWD), usually used only as a supplement to SSD, DTD, or SMD to better mitigate
vapors found to be migrating through the wall.
Step 3: Design Building Mitigation System
EPA recommends the final detailed design of the selected vapor intrusion mitigation technology specify the number
and location of suction points, location and size of piping, suction fan, piping networkand exhaust system, and
sealing options to be used in conjunction with the ADT technology. Fre-mitigation diagnostic testing can provide
information about the suction field underneath a building and pressure differences that will need to be overcome
(EPA 1993a) if the ADT system is to be effective. Dagnostic testing during installation can also help verify the
adequacy of the design.
Step 4: Install Building Mitigation System
EPA recommends that the vapor intrusion mitigation system be installed consistent with design specifications by
equipment manufacturers, local permit conditions and regulations, and relevant industry standards.
Step 5: Confirm the Installed System is Operating Properly
EPA recommends a visual inspection of the installed system as a routine quality assurance step to confirm that all
construction details have been completed. Post-construction monitoring is recommended (referto Section 8.4) to
demonstrate the ADT system is operating appropriately and effectively. Where a vapor intrusion mitigation system is
not performing adequately, post-construction diagnostic tests can be helpful in trouble-shooting (EPA 1993a).
Step 6: Ensure Proper Operation and Maintenance of Vapor Intrusion Mitigation System (referto Sections 8.3
and 8.4)
EPA recommends proper system maintenance and periodic inspections and monitoring to ensure the system is
operating as designed and is effective at reducing indoor air concentrations to (or below) target levels. EPA
recommends that site managers provide the building owner/occupant with information to help ensure proper
operation and maintenance of the system.
EPA recommends that periodic inspections include periodic measurements to confirm that the building mitigation
system is continuing to perform adequately.
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The U.S. Navy has issued a concise fact sheet that contains useful technical information (DoN
2011b).
8.2.3 Approaches and Considerations for New Buildings
The ADT systems described above are generally available for new buildings also. However, a
wider array of approaches and technical options is typically available to mitigate or avoid vapor
intrusion for new buildings, compared to existing buildings. These options potentially include the
choice of building location and opportunities to modify the building design and construction,
which are not available for existing buildings. For example:
At some sites, contaminated areas most likely to produce unacceptable vapor intrusion
exposures can be avoided and designated for another purpose, such as recreational
space or undeveloped landscape.
Mitigation needs can also be considered in the selection of heating and cooling systems,
which are normally selected based only on economics, aesthetics, preference, and
custom. A system design that avoids creating under-pressurization inside the structure
and maintains over-pressurization inside the structure may be effective in mitigating
vapor intrusion.
Passive barriers, such as a low-permeability membrane, can be more readily installed
between the soil and the building during new building construction. Passive barriers are
intended to reduce vapor intrusion by limiting openingsfor soil gas entry. However,
passive barriers as stand-alone technologies may not adequately reduce vapor intrusion
owing to difficulties in their installation and the potential for perforations of the barrier
during or after installation. They are commonly combined with ADT systems or with sub-
membrane ventilation systems to help improve their efficiency.
Venting layers can be more readily installed between the soil and the building during
new building construction.224
New buildings may be designed to include a highly ventilated, low-occupancy area at
ground level, such as an open parking garage.
Steps 2-6 of the Vapor Intrusion Mitigation Quick Guide provided in Table 8-1 are also pertinent
to newly constructed buildings. EPA guidance for selecting, designing, and installing vapor
intrusion mitigation systems for new buildings can be found in several publications (e.g., EPA
2008c). The U.S. Navy has issued a concise fact sheet that contains useful technical
information (DoN 2011c).
224
Constructed sub-slab ventilation systems typically consist of: a venting layer (e.g., filled with porous media such
as sand or pea gravel; or suitably fabricated with continuous voids) below a floor slab to allow soil gas to move
laterally to a collection piping system for discharge to the atmosphere; and a sub-slab liner that is installed on top of
the venting layer to reduce entry points for vapor intrusion. These and other sub-slab ventilation systems function by
drawing outside air into and through the sub-slab area, which dilutes and reduces concentrations of vapor-forming
chemicals, and provides a route for soil gas to vent to the atmosphere or migrate outside the building footprint, rather
than into a building.
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8.2.4 Owner/Occupant Preferences and Building Access
Building owners and occupants can initially be notified in various ways that their home or
building warrants construction and operation of a building mitigation system. Section 9.5
provides information regarding such notifications and other messages pertaining to building
mitigation.
Whereas EPA managers and mitigation system designers may be primarily concerned with the
performance, cost-effectiveness, and reliability of any mitigation system, the building owners
and occupants may have additional perspectives and opinions that warrant consideration during
technology selection, design, construction, and operation. For example, owners and tenants will
often have strong opinions about where fans and piping are located, what level of fan noise is
acceptable, and what quality of construction craftsmanship is satisfactory. When there are
multiple mitigation options (for example, at a large commercial building), EPA recommends
these options be presented fairly to the building owner and tenants, explaining the advantages
and disadvantages associated with each and describing the rationale for the preferred
alternative.
In some cases, obtaining and scheduling access to a building can be difficult, whetherthe
structure is a commercial or institutional building or a private residence. Commercial building
tenants may not want construction activities disrupting business operations. Some
homeowners/tenants may resist granting access to their home. Other homeowners/tenants may
prefer to schedule tests before or aftertheir work-day. To address these practical and logistical
concerns, EPA recommends that an access agreement(s) be executed between the property
owner, any tenants, and the mitigating entity to ensure appropriate access as needed to
operate, maintain, and monitorthe engineered exposure controls in each impacted building.
8.3 Operation and Maintenance ofVapor Intrusion Mitigation Systems
For purposes of this Technical Guide, operation and maintenance (O&M) is used generically to
refer to periodic inspections, component maintenance or replacements, repairs, and related
activities that are generally necessary to ensure continued operation and effectiveness of
engineered exposure controls to mitigate vapor intrusion. EPA generally recommends that such
O&M activities be conducted routinely, be documented in an O&M plan (as described further in
Section 8.5), and consider recommendations of equipment manufacturers, if any, and site-
specific factors. Additional information about ensuring continued effectiveness of systems is
available in EPA (2009b).
Design specifications for vapor migration systems may include (1) a maintenance frequency that
varies over the operating period of the mitigation system and/or (2) a provision to evaluate and
modify the frequency based on data or information obtained during monitoring and
maintenance. For example, it may be acceptable to reduce inspection or maintenance
frequency once efficient system operation has been demonstrated for at least an initial year,
with triggers for additional, unscheduled inspections following alarms (from warning devices)
and floods, earthquakes, and building modifications, if any.
Typical O&M activities for either passive or active systems may include, but are not limited to:
Routine inspection of all visible components of the vapor intrusion mitigation system,
including fans, piping, seals, membranes and collection points, to ensure there are no
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signs of degradation or blockage. EPA recommends that the as-built drawing for the
vapor intrusion mitigation system be examined to verify the system configuration has not
been modified.
Visual inspection of the building to evaluate whetherany significant changes were made
(such as remodeled basement, newfurnace) that would affect the design of the vapor
intrusion mitigation system or the general environment in which it is operated. A crawl
space SMD membrane, for example, may warrant repair or replacement if its integrity is
compromised.
Visual inspection of the area of concern (including basement floor and wall seals,
sumps, floor drains and utility penetrations) to ensure there are no significant changes in
conditions that would warrant modification of the system design.
Routine monitoring of vent risers for flow rates and pressures generated by the fan to
confirm the system is working and moisture is draining correctly.
Routine maintenance, calibration and testing of functioning components of the venting
system consistent with the manufacturers' specifications.
o Pressure readings for both active and passive depressurization systems as well
as positive pressurization systems (e.g., periodic verification of measurable
pressure differences across the slab).
o Confirmation that the extraction fan is operating.
o SSD system fans generally can function well for prolonged periods without
maintenance; however, EPA recommends fans be replaced periodically
throughout the operating life of the system (e.g., every 4 to 10 years) to avoid
breakdowns and associated problems.
Inspection of external electrical components to identify undesirable conditions, such as
excessive noise, vibration, moisture, or corrosion, and to verify that the fan cut-off switch
is operable.
o Inspection of the fan(s) is important throughoutthe operating period but may be
particularly important near the end of its expected lifespan. Noisy fans typically
indicate problems with ball bearings and warrant replacement on that basis.
o Confirmation of adequate operation of the warning device or indicator.
Confirmation that building owner/occupants are knowledgeable about howto maintain
system operation. Confirmation that a copy of the O&M manual is present in the building
and has been updated as necessary.
EPA also recommends that the site team determine if there has been any change in
ownership/occupant. If such a change has occurred, EPA recommends the site manager work
with the new owner/occupant to ensure continued integrity and operation of the vapor intrusion
mitigation system.
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8.4 Monitoring of Vapor Intrusion Mitigation Systems
Regardless of the type of system selected for mitigating vapor intrusion in a given building, EPA
recommends monitoring to demonstrate that performance standards are achieved at the time of
installation and that those performance standards continue to be met throughout the operating
period of the mitigation system. EPA recommends that any monitoring program developed for a
building mitigation system be based upon site-specific considerations, including the degree of
risk or hazard being mitigated, the building use, the technology used to mitigate vapor intrusion,
whether the subsurface vaporsource(s) is stable in extent and concentration, and coordination
with site remediation efforts. For example:
An older building with highly volatile chemicals at high concentrations may need a more
intense level of monitoring than a new building with lower concentrations of less volatile
chemicals.
Passive systems are generally less predictable and less efficient at preventing vapor
intrusion than active systems and, therefore, typically warrant more intensive monitoring,
all else being equal.
When contaminated groundwater plumes are migrating to newareas (i.e., expanding) or
concentrations in shallow groundwater are increasing, increased frequency and intensity
of mitigation monitoring may be warranted.
During start-up, some remediation methods have the potential to alter soil gas conditions
in ways and to a degree that may be difficult to predict. Increased frequency and
intensity of mitigation monitoring may be warranted when such remediation methods are
implemented near buildings undergoing mitigation.
Mitigation monitoring will generally entail two phases: (i) an initial post-construction phase,
which is generally more intensive; and (ii) a subsequent phase, which may be comprised of
fewer diagnostic tests to be conducted periodically. As with radon mitigation systems (EPA
1993a, Section 11.1.2), results of indoor air sampling during initial post-construction monitoring
may be used to demonstrate that the occupant's exposure to vapor-forming subsurface
contaminants has been reduced as anticipated. In addition, pressure field measurements in the
subslab region can be used to demonstrate thatthe system has attained hydraulic control and
communication (e.g., depressurizationin the case of an ADT system) over the footprint of the
building (or portion of a large building, as appropriate, considering the extent of subsurface
contamination). Adjustments to the mitigation system and/or additional diagnostic testing (EPA
1993a, 1993c) may be warranted if the results of such testing do not clearly demonstrate that
the system is achieving its intended performance and effectiveness. Once an adequate
demonstration of effectiveness has been made for the vapor intrusion mitigation system,
periodic monitoring is recommended to verify thatthis performance is sustained; for this
purpose, monitoring may be comprised of fewer types of tests than during the immediate post-
construction (i.e., start-up) phase at the discretion of EPA when the subsurface vaporsource(s)
is stable. Examples of various monitoring scenarios for these two phases are provided in Table
4 of CalEPA (2011), Table 6-2 of NJDEP (2012), and Table 3-1 of MADEP (2011). Additional
information about ensuring continued effectiveness is available in the Operational and
Functional Determination and the Transfer of Fund-lead Vapor Intrusion Mitigation Systems to
the State (EPA 2009b).
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When monitoring to assess the performance and effectiveness of building mitigation
technologies, EPA recommends employing the methods, approaches described in Sections 5.4,
6.2, 6.4 and 7.0 of this Technical Guide. EPA also recommends that monitoring programs that
assess the performance and effectiveness of remediation and mitigation systems be
documented, preferably in work plans similar to those recommended herein for characterizing
and assessing the vapor intrusion pathway (see Section 6.2). Such vapor intrusion monitoring
plans may be incorporated as part of a comprehensive remedial design and operations manual
or as a stand-alone document, depending upon site-specific circumstances. In addition, EPA
recommends that data and other results obtained through such monitoring programs be
documented (e.g., in the administrative record), as they become available.
The remainder of this section identifies and further discusses some elements commonly
incorporated in monitoring programs for active depressurization technologies.
Pressure Measurements
Sub-slab probes can be used to monitor differential pressures for a direct indication of the
hydraulic performance of ADT systems (i.e., the pressure difference across the slab prevents
soil gas entry); see Section 2.3. For basements, the walls that are underground become part of
the critical building envelope that must prevent soil gas entry. For subsurface depressurization
systems, EPA recommends that the pressure gauge be monitored quarterly to verify the system
is operating efficiently. A reduced monitoring frequency may be appropriate after one year of
successful operation of the remedial system.
Tracer Testing
Openings within the building or leaks in the mitigation system can affect system performance.
Tracers can be used either for leak detection through barriers, building materials or system
components (piping, for example) or to measure the air exchange rate in the building.
Smoke testing is a qualitative form of tracer testing used to detect leaks (e.g., at seams and
seals of membranes in SMD systems or at potential leakage points (openings) through floors
above sealed crawl space systems or through conduits that facilitate preferential vapor
migration), or to test airflow patterns. A limitation of smoke testing in existing structures is that
non-noxious smokes can be expensive, and cheaper high-volume smoke sources can leave
undesirable residues. The efficacy of smoke testing in some applications has been questioned
on the grounds that many leaks are too small for visual detection using this method (Maupins
and Hitchins1998, Rydock2001), and that leaks large enough to detect using smoke could be
detected other ways. More quantitative methods have been recommended, such as tracer
testing with instrumentation for quantitative results.
Air Sampling
Once an adequate demonstration of vapor intrusion mitigation system effectiveness has been
made, indoor air quality generally will be acceptable as long as an adequate pressure difference
is maintained throughout the footprint of the building. Periodic or intermittent sampling of indoor
air, nevertheless, warrants consideration, since indoor air data can provide direct confirmation
that the system is reducing exposure levels of vapor-forming chemicals and because
depressurization technologies can be expected to alter the distribution of vapors in the vadose
zone and available for soil gas entry, if any.
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Weather-Related Considerations
EPA recommends that weather conditions be noted during monitoring activities (EPA 1993a).
Weather conditions, such as temperature and precipitation, can affect the performance of a
vapor intrusion mitigation system. Forexample, cold temperatures may increase the building
depressurization created by the thermal stack effect and thus increase the driving force for soil
gas entry, depending upon the height of the house and the temperature difference between
indoors and outdoors (see Section 2.3). As a result, the ADT system may need to overcome
more building depressurization than originally considered when designed. Precipitation may
also increase moisture in the fill under the slab, which may affect the performance of the
system, and is a factor to consider in developing a monitoring program.
Alarms
Alarms generally are used as part of a monitoring program to ensure that malfunctions of vapor
intrusion mitigation systems are timely and readily detected and addressed. According to ASTM
(2003), "All active radon mitigation systems shall include a mechanism to monitor system
performance (air flow or pressure) and provide a visual or audible indication of system
degradation and failure" (i.e., an 'alarm'). ASTM goes on to say, "The mechanism shall be
simple to read or interpret and be located where it is easily seen or heard. The monitoring
device shall be capable of having its calibration quickly verified on site." Such devices may
indicate operational parameters (such as on/off or pressure indicators) or hazardous gas
buildup (such as percent LEL indicators). EPA concurs with the cited advice from ASTM and
recommends it be considered when monitoring and maintaining mitigation systems for vapor-
forming chemicals and sites addressed by this Technical Guide.
In particular, EPA recommends that system failure alarms be installed on active
depressurization systems, and appropriate responses to alarms be communicated by the
building owner/occupants. EPA also recommends that alarms be placed in readily visible,
frequently trafficked locations within the respective building and their proper operation be
confirmed on installation and monitored periodically.
Placards
EPA also recommends that permanent placards be placed on the system to describe the
system's purpose and operational requirements (e.g., power source) and instructions on what to
do if the system does not operate as designed (for example, a phone numberto call for
corrective action). EPA recommends the placard provide information about how to read and
interpret the monitoring instruments or warning devices provided. EPA also recommends that
these placards be placed as close to the monitoring/alarm part of the system as possible, as
well as close to the fan or other active parts of the system.
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8.5 Doc ume ntatio n o f E ng ine e re d Ex po s ure C o ntro Is fo r Vapo r Intrus io n M itig atio n
EPA recommends that documentation be provided to building owners and occupants and
appropriate regulatory agencies225 describing the vapor intrusion mitigation system (i.e., a
'system manual') and its associated O&M (i.e., an 'O&M plan'). The system manual provides a
detailed record about the mitigation system, including as-built drawings, permits (if any), copies
of agreements, and construction/layout plans, whereas the O&M manual describes the O&M
activities to be conducted routinely and identifies which party is responsible for these O&M
activities. Additional information about ensuring continued effectiveness is available in
Operational and Functional Determination and the Transfer of Fund-lead Vapor Intrusion
Mitigation Systems to the State (EPA2009b).
O&M Plan
O&M plans generally are prepared on a site-specific basis, and they often are particularly
useful at sites where:
Monitoring is needed to verify remedial effectiveness.
The remedial system warrants periodic adjustments and maintenance.
Human health risk would result if the system fails or if site conditions change.
Conditions that would trigger specific contingent response may occur sporadically or
episodically.
Some site remedial systems may also warrant the use of a regulatory agency-approved
contingency plan or similar corrective response document approved by the regulatory
agency to identify conditions that may triggerthe need for additional maintenance, collection
of additional data, modifications of monitoring frequency, or other responses to ensure the
remedy remains effective.
Communication with building owners and occupants about vapor intrusion and the O&M of a
vapor intrusion mitigation system is critically important. Forexample, building owners may
be concerned about some aspect of system operation and decide to turn it off. It is important
to communicate that turning off the system may result in harmful indoor air concentrations
inside the building.
System Manual
The specific contents of the system manual will depend on the type of system. EPA
recommends, however, that the system manual generally include at least the following
information or items:
Cover/transmittal letter;
For example, EPA recommends the potentially responsible party (PRP) provide a system manual and O&M plan
to EPA at PRP-lead Superfund sites.
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Description and diagram of final as-built system layout with components labeled;
Building permits for a vapor intrusion mitigation system;
Pre- and post-mitigation air and gas sampling data;
Pre- and post-mitigation diagnostic test data;
Copies of contracts and warranties;
Proper operating procedures of the system;
Contact information of the contractor or installer;
Copy of signed access agreement;
Copy of vapor mitigation system O&M agreement;
Copy of pre-mitigation sample result letter (see Section 9.4);
Copy of post-construction sample result letter;
Contact information in case of future questions; and
Inspection and maintenance guidelines.
User's Guide
Documentation typically is also provided to the property owner and occupant in the form of a
user's guide suitable to keep lay persons informed about the system and to provide a summary
reference in case questions or issues arise pertaining to the system.
A user's guide is a brief summary of why a vapor intrusion mitigation system was installed at a
property and how the system works, and may include the following: (1) a brief description of the
system and its proper range of operation; (2) contact information for the party responsible for
responding to malfunctions and ensuring the system performs properly; and (3) information
about routine maintenance to be conducted by the owner/occupant, if any. EPA recommends
that a user's guide be placed near the system for quick access and easy reference (e.g., into a
clear protective sleeve and attached to the main extraction pipe of the ADT system). An easy-to-
read user's guide may be especially helpful at rental properties because the guide informs each
new tenant about what the system is and why it was installed.
8.6 Use of Institutional Controls
ICs may be used to restrict certain land uses, buildings, or activities that could otherwise pose
an unacceptable human exposure via the vapor intrusion pathway.
Response actions for vapor intrusion may include ICs to restrict land use for protection of
human health regardless of whether a vapor intrusion mitigation system provides interim
measures to control (i.e., reduce, limit) human exposures. ICs can be used as either an interim
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response until site cleanup goals are reached or as part of a long-term response where vapor-
forming waste remains in place.
General EPA guidance on ICs is provided in Institutional Controls: A Guide to Planning,
Implementing, Maintaining, and Enforcing Institutional Controls at Contaminated Sites ("PIME 1C
Guidance") (EPA 2012d). As discussed in the PIME 1C Guidance, ICs are non-engineered
instruments, such as administrative or legal controls, that help to minimize the potential for
human exposure to contamination and protect the integrity of a response action. ICs typically
operate by imposing land or resource use restrictions at a given site or by conveying notice to
stakeholders regarding subsurface contamination or the possible need to refrain from certain
actions that may result in human exposure to hazardous chemicals. For example, ICs may be
used to restrict the development and use of properties for certain land uses (e.g., prohibiting
residential housing, hospitals, schools, and day care facilities).
In some situations, ICs can be used to restrict access to a property, facilitate response activities
conducted by a responsible party or EPA, such as the installation or maintenance of vapor
intrusion mitigation systems, or help ensure the integrity of vapor mitigation systems. ICs may
also be used to help inform the need for vapor intrusion mitigation for future construction where
vapor-forming waste remains in place and may pose unacceptable human health risk due to
vapor intrusion.
As described further in Section 2.2 of the PIME 1C Guidance, ICs can be described in four
general categories:
Proprietary controls.
Governmental controls.
Enforcement and permit tools with 1C components.
Informational devices.
The first three categories (i.e., proprietary controls, governmental controls, and enforcement and
permit tools with 1C components) typically memorialize and prescribe substantive use
restrictions concerning the land or resource use, while informational devices generally operate
to provide notice of contamination and any remedial activities to parties. Depending on the
nature of the site and the particularjurisdiction in which it is located, certain instruments may not
be available or feasible for a particular site. Certain ICs may help facilitate how interim response
actions and subsurface remediation are carried out, such as provisions addressing access,
O&M of vapor intrusion mitigation systems, and design specifications for buildings (see Example
#3 box below).
8.6.1 Evaluating ICs in the Overall Context of Response Selection
As a site moves through a program's response selection process (for example, a Superfund
remedial investigation/feasibility study [RI/FS] or RCRA facility investigation/corrective
measures study [RFI/CMS]), EPA recommends that site managers develop information about
reasonably anticipated future land uses and infer reasonably expected exposure pathways
related to land use. This information may be incorporated in the conceptual site model and often
can be used to evaluate whether ICs will be needed to ensure protectiveness of current and
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reasonably anticipated future land uses over time. EPA's land use guidance (EPA 1995a,
201 Oc) recommends that the site manager discuss reasonably anticipated future land uses of
the site with local land use planning authorities, local officials, property owners, and the public,
as appropriate, as early as possible during the scoping phase of the RI/FS, RFI/CMS, or
equivalent phase under othercleanup programs.
EPA recommends that the Region's decisions to implement ICs be documented in proposed
cleanup plans and in final cleanup decision documents. For example, for CERCLA cleanups,
the proposed restriction, and need for ICs would normally be identified in the Proposed Plan for
notice and opportunity to comment by potentially affected landowners and the public. Such use
restrictions or notices typically are then selected and memorialized in the Record of Decision
(ROD).
In some cases, unanticipated changes in land use may occur after the response action is
implemented, which may impact the protectiveness of a completed response action and raise
questions concerning the effectiveness of the ICs. As a result, vapor intrusion may be identified
as a potential human exposure pathway in a subsequent periodic review. In this case, EPA
recommends that site managers evaluate options for modifying the original response decision,
including the need for newor additional ICs consistent with existing and reasonably anticipated
future land uses and other response selection considerations.
8.6.2 Common Considerations and Scenarios Involving ICs
The evaluation of whether an 1C is needed at a contaminated site, including one where the
vapor intrusion pathway poses a current or potential threat to human health, is a site-specific
determination. When evaluating whether an 1C will be needed, EPA recommends that EPA
Regional staff consider whether the site meets unlimited use and unrestricted exposure
(UU/UE), among other factors. UU/UE is generally the level of cleanup at which all exposure
pathways present an acceptable level of human health risk for all land uses, including
reasonably anticipated future land use scenarios that are considered during response selection.
Common scenarios where ICs may be a useful instrument for fostering protectiveness at a site
involving vapor intrusion threats include, but are not limited to, the following:
1. Existing buildings overlie soil or groundwater contamination, or a migrating groundwater
plume that is moving toward existing buildings potentially poses a future vapor intrusion
threat;
2. Future construction is planned or is reasonably anticipated on a site that overlies
subsurface contamination with vapor-forming chemicals;
3. Changes to building construction/design (such as remodeling or ventilation changes) or
building use (such as commercial building converted for residential use) potentially affect
exposure to the vapor intrusion pathway;
4. Vapor intrusion mitigation systems are needed in buildings, or existing ventilation
systems are being utilized for vapor intrusion mitigation, and continued access is sought
to facilitate their O&M;
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5. Response actions to reduce source contamination will not immediately meet response
objectives; and
6. Response actions to reduce or eliminate source contamination will not be taken (for
example, where it is technically impracticable to treat groundwaterthat is the source of
vapor intrusion).
Informational ICs may also serve to provide notice to parties, including prospective purchasers,
about what land or building uses are compatible with human health risk that may be posed by
vapor intrusion at the site. For example, modifications to a building's ventilation or air
conditioning system may affect building under-pressurization in a way that fosters a greater
potential vapor intrusion threat. Various ICs can be tailored to address construction and design
specifications of both existing and future buildingsa local ordinance, for example, may require
parties to submit a building design to its building departmentthat incorporates mitigation
measures as determined appropriate by a Professional Engineer (P.E.) (see 1C Example #1).
In addition to restricting land, building, or resource use, some types of ICs may provide an
effective means for addressing O&M at vapor intrusion sites consistent with decision documents
and enforcement documents. This could happen, for instance, when an 1C specifies that
mitigation systems be installed and maintained in future construction or if the use of an existing
building changes (e.g., industrial building use changes to mixed commercial or residential uses).
Provisions regarding access to and periodic maintenance and testing of the mitigation systems,
and other site-specific obligations may be incorporated into the 1C (see 1C Example #2).
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1C EXAMPLE 1:
CityofMandan, North Dakota Ordinance No. 1002 (City of Mandan 2006)
In 2006, the City of Mandan, North Dakota, enacted an ordinance that created an Environmental
Institutional Control Zoning Districtto define an area of downtown Mandan impacted by petroleum
contaminated soil and groundwaterandto establish ICs for the protection of human health and the
environment. Among other provisions, the ordinance requires any person proposing
redevelopment, demolition, excavation, grading, or construction activities at properties within the
District to submit to the city administrator ortheir appointee a contingency plan, approved by the
North Dakota Department of Health, to evaluate and manage any petroleum contaminated soils or
groundwater and any potential petroleum vapor impacts. The contingency plan must be prepared
by a P.E. with experience in the environmental field, and the plan must consider and protect
against, among otherthings, the vapor intrusion pathway. In addition, the ordinance also provides
for restrictions on construction of new structures within the District. In pertinent part, the ordinance
provides:
"Any person proposing to construct a new structure within the District shall submit a design for that
structure that incorporates engineered controls to mitigate the effects of the potential presence of
petroleum in the subsurface to the city administrator or their appointee. The design must be
prepared by a P.E. and the design must be approved by the North Dakota Department of Health
and must meet additional applicable codes and standards relative to the presence of petroleum.
The design shall protect the public health and the environment by considering, at a minimum a)
historic water/product intrusion; b) historic petroleum vapor/odor issues; c) potential future
water/product intrusion; and d) potential future petroleum vapor/intrusion. The design shall
incorporate vapor barriers, venting system, groundwater suppression/collection, and specialized
HVAC as determined appropriate by a P.E."
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1C EXAMPLE 2: State 1C Legislation
Some states have enacted statutes that directly authorize proprietary controls for the purpose of
preventing use in conflict with environmental contamination or remedies. These state statutes
divide into ones modeled after the Uniform Environmental Covenants Act (UECA)226 and other
non-UECA statutes.227 These UECA and non-UECA state statutes tend to provide advantages
over traditional common law proprietary controls by reducing certain legal and management
complications associated with their use. The Model UECA, for instance, contemplates that the
grantee or "holder" of the "environmental covenant" may be given specific rights or obligations with
respect to future implementation of the environmental covenant.228 This ability to oblige parties to
undertake affirmative actions at a site, such as long-term maintenance of a cap orO&M of a vapor
intrusion mitigation system, through a UECA environmental covenant, is different from traditional
common law proprietary controls.
Proprietary controls that bind current and subsequent landowners (that is, the proprietary control
"runs with the land") to use restrictions at properties, as well as oblige them to undertake
affirmative obligations, may have utility at vapor intrusion sites. For instance, at a contaminated
site in Bucks County, Pennsylvania, an environmental covenant executed pursuant to the
Pennsylvania Uniform Environmental Covenants Act contained provisions to address vapor
intrusion threats. In addition to provisions for access, annual inspections, compliance reporting,
and other specifications related to cleanup activities, parties to the environmental covenant agreed
to construct slab-on-grade buildings without basements and install vapor barriers as an
engineered control to mitigate the potential for vapor intrusion as part of the eventual development
of the property. Further, the environmental covenant provided that engineering plans for the vapor
barriers first be submitted to and approved by EPA prior to construction. For examples of
environmental covenants executed pursuant to the Pennsylvania Uniform Environmental
Covenants Act, Act No. 68 of 2007, 27 Pa. C.S. งง 6501-6517:
http://www.depweb.state.pa.us/portal/server.pt/communitv/land recycling proaram/20541/uniform
environmental covenants act/1034860
226 UECA was developed by the National Conference of Commissioners on Uniform State Laws. See:
www.uniformlaws.org.
227 See, for example, Colo. Rev. Stat. ง 25-15-320 (2011); Cal. Qv. Code ง 1471 (2011).
"Grantee" is a traditional property law term describing a person to whom property is conveyed. States that have
passed legislation based on UECA have created different legal concepts specific to those jurisdictions. For example,
UECA jurisdictions typically define "holder" and "environmental covenant" to reflect, respectively, the grantee and the
servitude that imposes the land or resource use restrictions. The model UECA provides that "[h]older means the
grantee of an environmental covenant..." See definition 6 in Section 2.0 of the model UECA.
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8.6.3 Selecting the Right Instrument(s)
When evaluating potential 1C instruments, EPA recommends that site managers and site
attorneys balance the relative advantages and limitations of 1C instruments under
considerationfor example, consider legal implementation issues, jurisdictional questions,
permanence and enforceability concernsand select those that best achieve the response
objectives. (1C Example #3 describes how these factors were considered at the Middlefield-Ellis-
Whisman Study Area.)
EPA guidance on ICs provides detailed considerations regarding the selection of ICs and the
relative strengths of the different categories of 1C instruments.229 Ultimately, the selection of ICs
is a site-specific evaluation based on the characteristics of the site (forexample, the nature and
extent of the vapor intrusion threat) and the particularjurisdiction in which it is located. There
are times when multiple 1C instruments can be "layered" to best ensure protectiveness of the
response action while meeting the response objectives outlined in the decision documents.230
Because many ICs are created pursuant to state and other non-federal laws, the authority to
implement and otherwise oversee these ICs resides with government entities other than EPA.
Units of local governments, for instance, typically have jurisdiction to implement, maintain,
enforce, and terminate certain governmental controls, such as zoning ordinances and building
permit conditions. Therefore, it is important to evaluate the capacity (financial, technical, etc.)
and willingness of the entity ultimately responsible for taking over 1C responsibilities prior to 1C
selection.231 Site managers and site attorneys are encouraged to coordinate early with 1C
stakeholders so that adequate assurances may be acquired and then subsequently maintained
as necessary over time.
Given the potential role of non-EPA entities, it may be appropriate for EPA to facilitate or
recommend a process by which 1C stakeholders provide similar assurances or otherwise reach
a common understanding232 regarding their respective 1C responsibilities to ensure that selected
ICs are effectively implemented, maintained, and enforced. At a vapor intrusion site, for
example, a zoning ordinance may be effective in preventing or ensuring responsible future
development of properties overlying a contaminated groundwater plume that presents a vapor
intrusion pathway threat. Such zoning ordinances generally are designed and enacted by the
local government. Once enacted, the ordinance must be followed and enforced for it to serve as
an effective 1C over its lifespan. One inherent limitation of governmental controls, however, is
that their implementation, modification, and termination generally followa legislative process
229
See Site Manager's 1C Guide and Section 3.2 of the PIME 1C Guide for a framework to consider when deciding
among available ICs.
230
See Section 3.2 of the PIME 1C Guide for more discussion on layering ICs.
231 See Section 3.8 of the PIME 1C Guide on 1C stakeholder capacity considerations.
232
Parties may be able to provide assurances or otherwise reach a common understanding regarding their respective
1C roles and responsibilities through various mechanisms that may be available under state law (forexample, a
Memorandum of Understanding, Memorandum of Agreement, Administrative Order on Consent, contract, City
Resolution, or enforceable agreement, etc.). For additional discussion about obtaining or memorializing 1C
assurances, see Sections 3.3, 3.8, and 4.3 of the PIME 1C Guide.
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1C EXAMPLE 3: Efforts to Address VI at the Middlefield-Ellis-Whisman Study Area
The Middlefield-Ellis-Whisman (MEW) Study Area is composed of four separate CERCLA sites
Raytheon Corp., Intel Corp. (Mountain View Plant), Fairchild Semiconductor Corp. (Mountain View
Plant), and portions of the former Naval Air Station Moffett Field Superfund siteand many
distinct parcels with land uses including residential, commercial, and light industrial. In 2009, EPA
finalized a Supplemental FS for the MEW Study Area that presented an evaluation of a variety of
remedial alternatives that could be used to mitigate potential vapor intrusion into current and future
buildings overlying the shallow plume of contaminated groundwater. The FS provided an analysis
of ICs using the NCP evaluation criteria: overall protection of human health and the environment;
long-term protectiveness and permanence; compliance with applicable or relevant and appropriate
requirements; reduction oftoxicity, mobility, or volume through treatment; short-term effectiveness;
implementability; and cost. The othertwo NCP evaluation criteria, state acceptance and
community acceptance, were evaluated in the ROD Amendment for the vapor intrusion pathway
remedy at the MEW Study Area.
In 2009, EPA published the Proposed Plan for the MEW Study Area that identified EPA's
preferred alternatives for the vapor intrusion remedy. The Proposed Plan identified the adoption of
a municipal ordinance as EPA's preferred 1C, but the City of Mountain View and concerned
property owners raised concerns that this was not necessary. Instead, EPA worked with the City
of Mountain View, California, to have the City formalize its permitting procedures that apply to
future construction. These permitting procedures oblige those proposing new building construction
within the MEW Study Area to obtain EPA approval of construction plans to ensure that, where
necessary, the appropriate vapor intrusion control system is integrated into building construction.
In a 2010 ROD Amendment, EPA presented its selected remedy for the vapor intrusion pathway
for the MEW Study Area. The ROD Amendment identified a combination of ICs for use at the site.
In place of a municipal ordinance as called for in the Proposed Plan, the ROD Amendment
selected reliance upon the internally modified permitting procedures by the City of Mountain
View's Building, Planning, and Permitting Departments. The City will also implement remedy
requirements for projects subject to the California Environmental Quality Act through that law's
procedures. With regard to existing commercial buildings where an active remedy is necessary,
EPA selected the use of recorded agreements that will help provide notice to current and future
owners and occupants, notice to EPA and the MEW Companies when there is a change in
building ownership or configuration, and the necessary access to install, maintain and operate the
vapor intrusion remedy. These agreements will be binding on and enforceable against future
property owners. Additionally, EPA selected the use of a tracking service to provide notice when
changes are made to properties within the MEW Study Area. Additional controls that will be
implemented by the City of Mountain View include creation of a mapping database to help ensure
that parties interested in properties within the MEW Study Area are informed of the appropriate
construction specifications when making inquiries with the City.
For more information on the MEW Study Area, see the Final Supplemental Feasibility Study for
the Vapor Intrusion Pathway (June 2009), Proposed Plan for the Vapor Intrusion Pathway (My
2009), and Record of Decision Amendment for the Vapor Intrusion Pathway (August 2010),
available at: www.epa.gov/reqion9/mew
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outside the authority of EPA that may raise questions regarding the reliability and continued
sustainability of the 1C. Obtaining early and continued assurances from a local government
specifying its commitment to the governmental control is recommended to help address this
limitation prior to its selection as part of a final cleanup plan.
Certain 1C instruments may not be available for use at a site, depending on federal, state, local,
tribal, or other applicable laws. Therefore, after determining the universe of ICs available for use
at a particular site, the practical and legal limitations can be evaluated. For example, large sites
with widespread contamination pose unique 1C challenges. These challenges could arise, for
instance, where a contaminated groundwater plume underlies many distinct parcels with
multiple property owners/occupants and vapor intrusion is the exposure pathway of concern.
Negotiating and implementing proprietary controls with many property owners, some of whom
may not be PRPs, may present legal, administrative, and otherchallenges.233
8.6.4 Long-term Stewardship
Long-term stewardship (LTS) activities are intended to help ensure that cleanups remain
protective of human health and the environment over time and that reuse activities remain
compatible with residual site contamination and associated human health risk potentially posed
by the vapor intrusion pathway. LTS procedures vary widely, but they generally are intended to
help assure compliance with the response actions at the site, including 1C compliance, by
providing relevant information in a timely manner to stakeholders who may use the property
(e.g., owners, excavators, developers, prospective purchasers or tenants) or to parties who
otherwise have 1C responsibilities (i.e., an entity with enforcement authority). LTS procedures,
for example, may entail provisions to monitor and then inform those responsible for the
response actions of potential changes in land use, ownership, tenancy, or building construction
at a site. Also, LTS procedures may facilitate monitoring IC(s) so that they remain effective and
reliable overtime. EPA guidance on ICs generally speaks to LTS procedures in terms of 1C
maintenance234 and enforcement activities.235
Periodic Reviews
A key part of 1C maintenance is a periodic process over the 1C life cycle to critically review and
evaluate the 1C instrument(s). Site managers and other stakeholders can evaluate the status of
1C implementation, maintenance and enforcement activities at a site and address any potential
1C deficiencies during the periodic review. The CERCLAFYR process, 236forexample, allows
site managers to evaluate overall protectiveness of the remedy, including ICs.237
ooo
See Section 4.4 of the RME 1C Guide for strategies for implementing proprietary controls.
234
The term "maintenance" generically refers to those activities, such as monitoring and reporting, that ensure ICs
are implemented properly and functioning as intended.
235
See Sections 8 and 9 of the PIME 1C Guide discussing 1C maintenance and enforcement activities.
236 See CERCLA section 121(c).
237 For general FYR guidance, see Comprehensive Five-Year Review Guidance (EPA 2001) at
www.epa.aov/superfund/cleanup/postconstruction/5vr.htm. For a more detailed discussion on 1C considerations
during the CERCLA FYR process, see Recommended Evaluation of Institutional Controls: Supplement to the
"Comprehensive Five-Year Review Guidance," (EPA 2011c).
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A list of possible IC-specific issues arising from any periodic review of a vapor intrusion site may
include:
ICs that are specified by the decision documents but are not yet in place;
ICs that are in place but are not attaining compliance with the use restrictions spedfied
by the decision documents (e.g., land use not compatible with IC-specified use
restrictions);
ICs are not identified in the decision documents but are necessary for the remedy to be
protective of human health because of the vapor intrusion pathway; and
Response selection assumptions change (e.g., toxicity values, potential exposure
pathways, or land uses change) and warrant the need for new or different response
actions, including additional IC(s).
1C Planning Documents
Responsibilities to monitor and report on 1C compliance, among other obligations, may be
documented in an Institutional Controls Implementation and Assurance Plan (ICIAP)238 or other
IC-related planning documents.239 An ICIAP can serve to: (1) document the activities necessary
to implement and ensure the long-term effectiveness and permanence of ICs (that is, the 1C life
cycle); and (2) identify the person(s) or organization(s) who, under state or local law, are
responsible for conducting those activities. Some ICs generally fall within the jurisdiction of a
particular category of stakeholders. Therefore, in addition to developing a comprehensive
planning document, such as an ICIAP, it may be useful for parties who share 1C responsibilities
(e.g., a PRP and local government regarding the use of governmental controls, such as an
ordinance or permitting system) to reach a common understanding and acknowledge various 1C
roles and responsibilities in a formalized manner. Where possible, EPA recommends that these
types of arrangements among 1C stakeholders be documented to describe commonly
understood roles and responsibilities for proper and effective monitoring, reporting, and other 1C
maintenance and enforcement activities.
8.6.5 Community Involvement and ICs
EPA recommends that site managers and site attorneys provide adequate opportunities for
public participation (including potentially affected landowners and communities) when
considering appropriate use of ICs (EPA 2012e). Those opportunities may include providing
appropriate notice and soliciting comments about cleanup plans. Community acceptance of the
need for ICs to provide protection from residual contamination and public understanding of the
legal and administrative steps for maintaining ICs often are important to the long-term
effectiveness of ICs.
For further guidance on developing ICIAFs, EPA developed Institutional Controls: A Guide to Preparing
Institutional Control Implementation and Assurance Plans at Contaminated Sites (EPA 2012e).
ooq
For example, other types of documents may address IC-related activities and responsibilities at a site, such as a
ROD, O&M plan, and land use control and implementation plan for federal facility sites.
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8.7 Termination/Exit Strategy
This sub-section focuses on the termination/exit strategy for vapor mitigation response actions.
Termination for vapor mitigation activities implemented underCERCLA, RCRA, Brownfields,
and federal facilities cleanups can occur when the objectives of these cleanup activities have
been met. For purposes of this sub-section, termination refers to the cessation of all activities
related to building mitigation, control of subsurface vapor source(s), ICs, and monitoring.
When mitigating vapor intrusion through subsurface source remediation, building mitigation, and
ICs, it is important to develop termination criteria, including the rationale fortheir selection, early
in the remedy planning (e.g., alternatives development) process. (Termination criteria generally
refer to numeric cleanup levels for each site-specific contaminant and narrative cleanup
objectives that are to be attained by the response actions.) EPA recommends that these
termination criteria be recorded in decision documents, in any other planning reports, and in
monitoring reports. EPA generally recommends also developing and documenting an exit
strategy, which clarifies how it will be determined that the termination criteria have been attained
(e.g., monitoring data and assodated statistics that will be used to demonstrate attainment).
This document could be developed in conjunction with the O&M plan and monitoring program
so that all stakeholders are provided with a clear and comprehensive set of termination criteria
for the remediation and mitigation systems and ICs. If site conditions (e.g., building usage,
vapor flux) change during the cleanup activities, it may become necessary to modify the
termination criteria and/or strategy.
When reviewing vapor intrusion activities, considerations for evaluating termination activities
may include:
Termination of subsurface remediation activities;
Termination of engineered exposure controls (building mitigation);
Termination of the associated ICs; and
Termination of monitoring.
8.7.1 Termination of Subsurface Remediation Activities
Where feasible, the preferred response to address vapor intrusion is to eliminate or substantially
reduce the level of volatile chemical contamination in the source media (e.g., groundwater and
subsurface soil) to levels that eliminate the need to mitigate or monitor vapor intrusion, as noted
in Section 8.1 of this Technical Guide. If subsurface remediation activities are being conducted
at the site, termination of these activities will be contingent on demonstrating that the chemical-
specific cleanup levels for the subsurface media have been attained. EPA recommends that the
termination criteria and exit strategy for these remediation activities be documented to foster
collection and evaluation of appropriate data to support eventual termination of these
subsurface activities.
EPA recommends that site-specific monitoring data be evaluated to determine if the termination
criteria have been met. Typically, monitoring will continue until the source(s) are remediated to
cleanup levels that eliminate the need to mitigate vapor intrusion at the point of exposure. As
appropriate, the exit strategy may provide criteria for phased remediation, resulting in a
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termination evaluation as source cleanup levels are achieved in parts of the contaminated area.
If the subsurface vapor source(s) is not remediated, it is generally anticipated that remediation
(and monitoring and any building mitigation) will continue.
Monitoring, in part, could be based on data similar to those that were used in a multiple-lines-of-
evidence approach for characterizing the pathway and human health risk or for supporting the
decision to undertake preemptive mitigation/early action (e.g., soil gas sampling, sub-slab
sampling, or vapor sampling within potentially affected structures). EPA recommends identifying
and documenting target concentration(s) that would allowfor remediation termination, along
with recommended monitoring/sampling frequencies.
If evaluation of the site-specific data indicates an increase in subsurface vapor concentrations
during the monitoring period, it may be appropriate to evaluate whether the subsurface
remediation plan and the CSM are adequate and appropriate.
Typically, once it is preliminarily determined that the subsurface remediation system(s) may be
terminated, EPA recommends a period of attainment monitoring. During the attainment period,
EPA recommends that the remediation system (e.g., reagent delivery equipment, soil vapor
extraction wells) not be operated for a sufficient period to allow subsurface vapors reach
equilibrium and indicate post-remediation conditions. The type and frequency of data collected
during attainment monitoring entails a site-specific determination. Additionally, EPA
recommends that criteria be described and documented, as part of exit strategy development, to
determine when ending the attainment monitoring period is appropriate. To develop an exit
termination strategy, site-spedficfate and transport data may be used to identify an appropriate
time period to allow the vapor concentrations to equilibrate. In addition, the termination of the
attainment monitoring period may involve an evaluation of the contaminant attenuation in the
vadose zone.
8.7.2 Termination of Building Mitigation
For purposes of this Technical Guide, "termination of building mitigation" refers to ending the
use of an engineered exposure control(s) that reduces or eliminates human exposure via the
vapor intrusion pathway. Typically, vapor mitigation is implemented when it is determined that
(1) unacceptable human health risk to inhabitants is identified, or (2) the system(s) was(were)
installed as part of an early action strategy (see Sections 3.3 and 7.8 for a discussion of building
mitigation as an early action).
As described in Section 8.2, vapor intrusion can be mitigated in specific buildings using either
an active or passive vapor mitigation system (or a combination thereof).
Active Building Mitigation
Generally, building mitigation systems are implemented in conjunction with the investigation and
remediation of subsurface vaporsource(s). Typically, building mitigation systems will be
operated until the source(s) are remediated to attain the cleanup levels (e.g., for the subsurface
vapor source(s)) that eliminate the need to mitigate vapor intrusion at the point of exposure. If
subsurface vapor source(s) are not remediated, it is generally anticipated that mitigation
activities will continue indefinitely. As appropriate, the termination strategy may provide criteria
for phased evaluation of system cessation as source cleanup levels are achieved in parts of the
contaminated area.
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Generally, once the subsurface vapor source(s) is remediated to levels that meet the remedial
objectives and protect human health from the vapor intrusion pathway, EPA recommends that
the site-specific monitoring data be evaluated to determine if the termination criteria for the
building mitigation system have been met. These monitoring data, in part, could be based on
data similar to those that were used for characterizing human health risk or for supporting the
decision to undertake preemptive mitigation/early action during the vapor intrusion investigation
(e.g., sub-slab soil gas sampling and/or indoorair sampling). EPA recommends identifying and
documenting target concentration(s) that would allowfor system termination, along with
recommended monitoring/sampling frequencies. In addition, certain site-specific performance
assessment data (e.g., standpipe vapor sampling) may also warrant consideration to make this
determination.
Typically, once it is determined that the building mitigation system may be terminated, EPA
recommends a period of attainment monitoring. During the attainment period, EPA recommends
that the mitigation system (e.g., subslab suction wells or ventilation fans) be offline for a
sufficient period to allow vapors beneath the structure reach equilibrium and indicate post-
remediation conditions. The type and frequency of data collected during attainment monitoring
entails a site-specific determination. Additionally, EPA recommends that criteria be established
in the exit strategy to determine when ending the attainment monitoring period is appropriate.
To develop an exit termination strategy, site-specific fate and transport data may be used to
identify an appropriate time period to allowthe vapor concentrations to equilibrate. In addition,
the termination of the attainment monitoring period may involve an evaluation of the
contaminant attenuation in the vadose zone.
If the attainment criteria evaluation indicates that cleanup levels and objectives are not being
met, it may be necessary to continue or resume subsurface remediation and mitigation
activities. Once it is determined that the cleanup levels and objectives have been met, the active
components of the system may be removed from the building; on the other hand, the building
owner may elect to continue to operate the mitigation system under their own discretion and for
their own purposes (e.g., radon reduction and moisture control). Once the cleanup levels and
objectives have been met, all O&M and monitoring of active mitigation systems specified by
EPA can cease.
Passive Building Mitigation
The termination of passive vapor mitigation systems will typically be similar to the criteria
established for the termination of active mitigation systems. In summary:
Like active mitigation systems, passive mitigation systems are typically implemented in
conjunction with the investigation and remediation of subsurface vapor source(s).
Generally, once the subsurface vapor source(s) is remediated to levels that meet the
cleanup levels and objectives that will protect human health from the vapor intrusion
pathway, EPA recommends that the site-specific monitoring data be evaluated to
determine if the termination criteria have been met.
If the site-specific criteria evaluation indicates that cleanup levels and objectives are not being
met, it may be appropriate to evaluate the current system's effectiveness or the possible
application of an active mitigation system. Once it is determined that contaminant cleanup levels
and objectives have been met, all O&M and monitoring specified by EPA can cease. EPA
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generally does not have a need to seek removal of barriers or seals that comprise a passive
mitigation system as part of termination activities.
8.7.3 Termination oflCs
"Termination of ICs," as used in this Technical Guide, refers to discontinuing any and all ICs
specified by EPA because restrictions on land or resource use and/or notices and other
informational devices are no longer necessary to help ensure protectiveness of human health
(i.e., human health risk from exposures to vapor intrusion, if any, are expected to be acceptable
in the absence of all IC(s)). Generally, ICs are implemented in conjunction with the investigation
and remediation of source(s). It is anticipated that ICs selected and implemented will be needed
until (1) subsurface vaporsource(s) are adequately remediated, or (2) restrictions on land,
resource, or building use are no longer necessary based on current and reasonably anticipated
future exposure scenarios. Therefore, when developing a termination strategy for ICs that have
been selected as part of a response action, the strategy is typically based on data collected
from the affected media.
EPA recommends that the exit strategy consider and identify cleanup levels for the subsurface
vapor source(s). As long as the subsurface vapor source exceeds such cleanup levels, it is
generally anticipated that the associated ICs will continue. As appropriate, the termination/exit
strategy may provide criteria for a phased 1C termination evaluation as source cleanup levels
are achieved in parts of the contaminated area.
If the site-specific criteria evaluation indicates that terminating the ICs is appropriate, EPA may
conclude that site conditions no longer warrant ICs being used as part of the response action for
the vapor intrusion pathway. At this point, EPA could notify the appropriate entity(s), such as
local or state government, tribe, affected landowner, or responsible parties, in writing that EPAs
response objectives have been met and that the 1C need not be maintained. As such, EPAs
oversight of the IC(s) can cease.
8.7.4 Termination of Monitoring
For purposes of this Technical Guide, monitoring includes activities conducted to verify thatthe
vapor intrusion pathway does not pose a health concern to building inhabitants while
remediation and mitigation activities are underway and in the event that the remediation and
mitigation activities are terminated. "Termination of monitoring," for purposes of this Technical
Guide, refers to ending any monitoring that was needed to verify that no further response action,
including IC-related activity, is necessary to protect human health from indoor air exposures
posed by vapor intrusion. When developing termination criteria formonitoring, the decision is
generally based on data collected from all the affected media.
As noted above, monitoring is generally implemented in conjunction with the remediation of
subsurface vapor sources(s). EPA recommends that the exit strategy consider cleanup levels
for all contaminated media. Typically, monitoring will continue until the source(s) are remediated
to cleanup levels that eliminate the need to mitigate vapor intrusion at the point of exposure (i.e.,
allow building mitigation systems to be terminated). If the subsurface vapor source is not
remediated, it is generally anticipated that any assodated monitoring will continue. As
appropriate, the exit strategy may provide criteria for phased monitoring, resulting in a
termination evaluation as source cleanup levels are achieved in parts of the contaminated area.
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Exhaust Option a:
Exhautt Stack on
House Exterior
Exhaust Option 1:
Exhaust Stack^th rough
Houic Interior
Exhaust (Released
To Exhaust Fan
Mounted In Atllc
Exhoumt Fan
(Rated far
Exterior Use
or Enclosed]
Flexible
Coupling
Strapping (or
Other Support)
OpenHoto
(6"to1B"
Radius)
Slope Horizontal Leg*
Down toward Sub-Slab
Hole, to Permit Condensote
Drainage
Connection to Other
Suction Point(e),
H Any
1. Detail lot Interior and exterior
stacks thown In later figures.
2. Options for effectively
supporting horizontal :md
vertical piping runs shown in
Inter figures.
3. Detail shown (or piping
penetration* through slab is one
option ainon g several. Other
option* shown in later figure*.
4. Closing of various slab openings
will sometimes be important for
good SSD performance.
Sealant Around
Suction Pipe
House Air Thro ugh ^
Unclosed Opening*
Figure 8-1 Illustration of Sub-slab Depressurization (SSD) System
Note: Shows one example of how a sub-slab depressurization system might be constructed. In this case,
the example suction pipe has been inserted vertically downward through the slab from inside the house.
Two options are indicated for location of the exhaust fan: one exterior to the building; and the other within
an attic. In both of these cases, the exhaust gas stream is to be vented outside and above the building.
Source: EPA (1993a; Figure 1)
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Exhau*t Option 2:
Exterior Stack
Note*:
1 .
Exhaust Option 1:
Interior Stack
Figure depict* suction pipe installed
remote from sump. Suction pipe could
also be Installed through eump cover.
Detail shown for pipe penetration
through Blab and connection to drain
tile can vary. Other option* are
shown later.
Alternative *urnp cover de*Jgn* are
di*cu**od liter.
Option* for supporting horizontal and
vertical piping run* are chown In later
figure*.
6. Detail for interior and exterior stack*
I* *hown In later figure*.
Closing various slab openings,
especially the perimeter wall/floor
foJrrt, will ปomeUmซ be Important lor
good *ump/DTD performance.
Crodo Levrl
Hole in
Drain Tile
Hear Suction
Pipe1
Gasket (or
Silicon* Caulk)
Existing Interior
Drain Tile Loop
Circling House
Sump Liner
Submersible
Sump Pump
Figure 8-2 Illustration of Drain-tile Depressurization (DTD) System
Note: Shows one example of how a drain-tile depressurization system might be constructed. In this case,
tiles are shown draining to a sump in the basement, to which an air-tight cover is sealed. The example
suction pipe has been inserted vertically downward through the slab at a location remote from the sump.
Two options are indicated for location of the exhaust fan: one exteriorto the building; and the other within
an attic. In both of these cases, the exhaust gas stream is to be vented outside and above the building.
Source: EPA (1993a; Figure 3)
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Exhaust Option 2
Exhaust Released
Flexible
Coupling
Exhaust Fan
(Rated for
Exterior Use
or Enclosed)
r Veneer Gap
Exhaust Option 1
1. Closure of top block voids can be
V&ry Important to avoid degradation
of BWD performance and Increased
heating/cooling penalty caused by
excessive leakage of house air into
the system.
2. Options (or use of individual pipe
BWD as a supplement to SSD are
illustrated In a later figure.
To Exhaust Fan
Mounted In Attic
Strapping (or
Other Support)
CIoBo Top Voids
Close Major Mortar Cracks and
Notes in Wall
Basement Air Through Block Pores,
Unclosed Cracks, and Holes
6" Dirt.
Collection
Pipo
From Connection*
into Other Walls
Top Void
Brick Veneer
Grade Level
:-,." ;:V 7-v,''":. '"':. : '
.' ' ." -' '- ' Cnil r.nc . " '.'""'''..'. '''"
Soil Gas
Figure 8-3 Illustration of Block-wall Depressurization (BWD) System
Note: Shows one example of how a block-wall depressurization system might be constructed. In this
case, individual suction pipes are inserted into the \ฃ>id space in the basement wall, which are connected
to one or more fans. Source: EPA (1993a; Figure 5)
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Exhaust Option 2
Exhaust Released
Above Eave
Flexible
Coupling
Exhaust Fan
(Rated for
Exterior Use
or Enclosed)
Notes:
1. The specific configuration depicted for the pipe penetration
through the membrane is one of a number ol alternatives.
Other options are shown In a later figure.
2. The membrane seams must always be sealed near the suction
point. Sealing of more remote seams may not always be
necessary, but is advisable.
3. The membrane can often be effectively sealed against the
foundation wall using a continuous bead of property selected
sealant (urethane caulk for cross-laminated polyethylenes,
other adhesive for regular polyethylenes). Other options
for sealing the membrane against the wall are discussed In
text.
Exhaust Option 1
To Exhaust Fan
Mounted In Attic
Hollow-Block
Foundation Wall
Grade Leva)
.
'".'.-.'.'.
"v
Crawl
Space
Membrane Sealed
Against Wall
with Bead of
Caulk or
Adhesive1
Hose Clamp and
Caulk, Sealing
Membrane
Around Pipe
Penetration
Strapping (or Other
Support) Will Sometimes
Be Necessary
Connection to
Other Suction
Point(s), II Any
-*- Slope Horizontal Legs
Down Toward Membrane
PVC Suction Pip*
Somi-Rigid Plastic
Plate Resting on Top
ol the I-Htting, to
Prevent Membrane from
Being Sucked Into the
End* of the T-Flttlng
Membrane
Sealant
Dirt Floor In
Crawt Space
PVC T-FItting Under
Membrane, to Support
Pipe and to Help
Distribute Suction'
Adjoining Sheets of
Membrane Overlapped
by about 12 Inches
Sealed with Caulk or
Other Adhesive
Figure 8-4 Illustration of Sub-membrane Depressurization (SMD) System
Note: Shows one example of how a sub-membrane depressurization system might be constructed. In this
case, the example suction pipe penetrates the membrane overlying a dirt floor.
Source: EPA (1993a; Figure 6)
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9.0 PLANNING FOR COMMUNITY INVOLVEMENT
Communicating information about environmental risk is one of the most important
responsibilities of site managers and community decision-makers. Simply stated, risk
communication, whether written, verbal, or visual statements concerning risk, is the process of
informing people about potential and perceived hazards to their person, property, or community.
EPA recommends that human health risk be described in context, recognizing there are
personal, cultural and societal dimensions of risk. EPA also recommends providing advice about
risk-reduction behaviorand encouraging a dialogue between the senderand receiver of the
message. The best risk communication occurs in contexts in which the participants are informed
about risks they are concerned about, the process is fair, and the participants are free and able
to solve whatever communication difficulties arise. Risk Communication in Action: The Risk
Communication Workgroup (EPA 2007) is one of several resources available that explain the
elements of successful risk communication and describe communication tools and techniques.
Thus, community involvement is a key component of any site investigation or other EPA
response action. Members of the public affected by environmental contamination can be made
aware of what EPA is doing in their community and have a say in the decision-making process.
Stakeholder and community involvement is particularly important for sites with vapor intrusion
issues, in part because the exposure to toxic vapors may pose a significant human health risk
that is unknown to inhabitants (in the absence of mitigation systems), as they potentially arise in
homes, workplaces, schools, and places of commerce and gathering. Because of the potentially
intrusive nature of assessment and mitigation for vapor intrusion, stakeholder involvement is
important throughout the process.
EPA generally recommends that stakeholderand community involvement be conducted from
the earliest stage of the site assessment and risk assessment process, with on-going education,
two-way communication, and discussion throughout the entire process to create community
trust and acceptance. For example, EPA recommends initiating community involvement
activities as soon as possible after determining that vapor intrusion may exist at a particular site.
Informing the community about vapor intrusion concerns and plans to conduct an assessment,
including sampling, can be resource intensive. Thus, EPA recommends evaluating each project,
in coordination with appropriate state and tribal officials, to assess the level of stakeholder
interest and need for community involvement during various stages of the decision-making
process.
Public Participation and Risk Communication
A meaningful community involvement process is founded upon knowledge of effective public
participation and risk communication practices. Public participation refers to the full range of
activities that EPA uses to engage communities in the Agency's decision-making process. In
2003, EPA updated its Public Involvement Policy. 24ฐ Its foundation includes seven basic steps
to support effective public participation:
240 EPA Public Involvement Policy (2003): http://www.epa.aov/publicinvolvement/policv2003/index.htm.
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1) Plan and budget.
2) Identify those to involve.
3) Consider providing assistance.
4) Provide information.
5) Conduct involvement.
6) Review and use input and provide feedback to the public.
7) Evaluate involvement.
To help implement the steps, EPA developed a series of brochures241 on effective public
participation that outline howto budget for, plan, conduct, and evaluate public participation.
EPA Program-Specific Community Involvement Guidance and Recommendations
CERCLA and other EPA regulations242 identify specific community involvement activities that
are appropriate at certain points throughout the cleanup process. Specifically, in 2005, OSWER
published the Community Involvement Handbook243 (EPA540-K-05-003). The handbook
presents legal and policy motivations for Superfund community involvement and includes
additional suggestions for involving the community in the Superfund process. In addition, EPAs
Proposed Guidelines for BrownfieldsGrantsencourages applicants to describe their plans for
involving community-based organizations in site cleanup and reuse decisions.244 The Grant
Funding Guidelinesfor State and Tribal Response Programs for brownfields funding also
encourage programs to establish, at a minimum, "mechanisms and resources to provide
meaningful opportunities for public participation."245 In addition, in 1995, EPA promulgated the
RCRA Expanded Public Participation rule (60 FR 63417-34, December 11,1995)246 which
created additional opportunities for public involvement in the permitting process and increased
access to permitting information.247
241 EPA Public Involvement Brochures: http://www.epa.gov/publicinvolvement/brochures/index.htm
242 40 CFR ง300.155 http://edocket.access.gpo.gov/cfr 2003/iulqtr/pdf/40cfr300.155.pdf
EPA Superfund Community Involvement Handbook'.
http://www.epa.gov/superfund/communitv/cag/pdfs/ci handbookpdf
EPA Brownfields Grants website: http://www.epa.gov/brownfields/cleanup grants.htm
245 EPA Brow nfields State and Tribal Response Program Grants website:
http://www.epa.gov/brownfields/state tribal/fund guide.htm
Section 7004(b) of the Resource Conservation and Recovery Act provides EPA broad authority to encourage and
assist public participation in the development, revision, implementation, and enforcement of any regulation, guideline,
or program under RCRA.
247 EPA RCRA Public Participation Manual: http://www.epa.gov/osw/hazard/tsd/permit/pubpart/manual.htm
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At sites with vapor intrusion issues, EPA recommends that the site planning team (i.e., the
remedial project manager (RPM) or on-scene coordinator(OSC); community involvement
coordinator (CIC); risk assessor; the enforcement case team; EPA contractor; state, tribal, or
local agency staff; federal agency staff relevant to the site; or others) to consider the following:
Develop a community involvement plan (CIP) or update the existing CIP.
Learn about the site and the community to foster development of a CIP that highlights
key community needs, concerns and expectations.
Commit to ongoing, sustained communication activities throughout vapor mitigation and
site cleanup efforts.
Develop a communication strategy248 and conduct outreach to inform stakeholders about
the facts and findings pertaining to the site.
Obtain written permission, if appropriate and necessary, for building/property access,
and involve the property owner/occupant in identifying or removing potential indoorair
contamination sources, including inspection of residence and completing an occupant
survey.
Fully communicate and interpret sampling results, and evaluate mitigation options, if any
a re warranted.
Recognize preference of owners and occupants for confidentiality with regards to
property-specific data.
When considering the most effective community involvement strategies, EPA recommends that
its previous involvement be considered, as well as the existence of community or neighborhood
groups and the phase of the regulatory process in which vapor intrusion is being addressed.
Additional resources for planning and implementing effective community involvement activities
are discussed in Section 9.2: Communication Strategies and Conducting Community Outreach.
9.1 Developing a Community Involvement or Public Participation Plan
A CIP is a site-specific strategy to enable meaningful community involvement throughout the
cleanup process.249 CIPs specify EPA-planned community involvement to address community
needs, concerns, and expectations that are identified through community interviews and other
means. A CIP will enable community members to understand the ways in which they can
participate in decision-making throughout the cleanup process. That is, the CIP is a way for EPA
to plan for informing and involving the community in the cleanup process and can be a powerful
A communication strategy can be one component of a CIP, but it addresses a specific event, issue, or concern,
such as an emergency response to a release, or communicating risk at a site. The CIP, on the other hand, describes
an overall strategy for conveying information throughout the cleanup process at a site.
249
Community involvement plans available at: http://www.epa.aov/superfund/communitv/pdfs/toolkit/ciplans.pdf
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way to communicate EPA's commitment to listening and responding to community concerns,
and provide timely information and opportunities for community involvement.
The CIP is intended to be a "living" document and is most effective when it is updated or revised
as site conditions change. When developing the CIP document, EPA recommends that the site
planning team consider the following steps:
Describe the Environmental Setting and Cleanup Process
Describe the release and affected areas (the site). This includes information about the site,
its history, the key issues related to site contamination, and how vapor intrusion fits into
EPA's overall cleanup effort at the site.
Describe and Learn about the Community
Describe the community. The community profile is a description of the affected community
that summarizes demographic information and identifies significant subgroups in the
population, languages spoken, and other important characteristics of the affected
community, such as whether the site is located in an area with environmental justice
concerns or includes sensitive populations. EPA recommends that the community profile
also document information sources and describe how the profile was developed.
Learn about community needs, concerns and expectations: Issues of concern to residents
and business owners can be identified through community interviews, informal discussions
and interactions, local media reports, and other insights about the affected community.
Questions may include:
What are public perceptions and opinions of EPA and the cleanup process?
How do people want to be kept informed (i.e., mechanisms to deliver information)?
How do people want to be included in the decision-making process?
What are the perceived barriers to effective public participation?
Are there other sources of pollution that affect the community?
Have there been past experiences of mistrust or any unique concerns?
This information can be used to recommend any special services to be provided, including
technical assistance, formation of a Community Advisory Group, facilitation/conflict
resolution, or translation services.
Write and Compile the CIP
Once the site planning team has learned about the community, it is time to put the
information together in a way that will be useful to EPA and the community. In addition to the
site description, community description, and community needs and concerns, the CIP also
may include a reference listing of contacts (name, address, phone, email) useful for the
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community or the site planning team. EPA recommends that the contact list generally
include contact information for
The site planning team.
Community groups and community leaders.
Local elected officials.
Local, state, tribal, and federal agency staff relevant to the site.
Media contacts (including social media outlets and community journalists).
Others, as appropriate.
To ensure that the CIP is indeed informed by the community, EPA recommends that a draft
of the CIP be shared with the community, and their input and feedback be invited as it
evolves. Again, the CIP is intended to be a "living" document and is most effective when it is
updated or revised as site conditions change. In some cases, particularly when the CIP is
updated or revised for a FYR or where community interest is minimal, a short CIP outlining
EPAs plan for community involvement may be all that is needed. For most sites, EPA
recommends that the CIP be written to address the community directly, and their active
involvement be invited at each stage of the cleanup process.
9.2 Communication Strategies and Conducting Community Outreach
EPA recommends that community outreach activities be initiated as soon as possible after
determining that vapor intrusion may exist at a particular site. Informing and educating the
community includes distributing information and providing opportunities for EPA to listen to
community concerns. EPA recommends community outreach activities be tailored to the
community based on information gleaned from community interviews and other methods used in
developing the CIP. Public health officials from state, tribal, or local agencies may be helpful in
communicating risk information and answering questionsfrom the community.
Communication Strategies
Communication strategies are plans for communicating information related to a specific issue,
event, situation, or audience. They serve as the blueprints for communicating with the public,
stakeholders, or even colleagues. EPA recommends that communication strategies:
Outline the objective and goals of the communication.
Identify stakeholders.
Define key messages.
Pinpoint potential communication methods and vehicles forcommunicating
information and obtaining information from the community for a specific purpose.
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When developing a communication strategy, the first step is to determine why the
communication is necessary and define its desired objectives, and then to focus on defining the
audiences and howto reach them. Keep in mind that the demographics, knowledge, and
concerns of the audiences play an important role in defining the key messages. Once the key
messages are defined, the outreach vehicle can be determined.
Conducting Community Outreach
The site planning team likely will use several different outreach techniques during the course of
the cleanup process. When planning community outreach, EPA generally recommends that the
site planning team collaborate with internal and external partners, such as local, state, and tribal
officials and departments of health; faith-based organizations; and community groups. It is
important to accommodate hearing-impaired or limited English proficiency (LEP)250 persons in
all outreach efforts by providing spoken or sign language interpreters at meetings and
translating printed outreach materials. It also is important to ensure that the community
understands the concept of vapor intrusion.
Examples of community outreach techniques to consider are described below.
Public Meetings/Gatherings
Public meetings are a useful opportunity to explain environmental conditions at the site,
potential health impacts, intended indoor air sampling, and remediation strategies. It may be
helpful to hold meetings prior to and following key sampling events to describe sampling
strategies and consequent results, respectively. EPA recommends that the meeting include
a period to address specific questions from the public regarding sampling results or any
other specific concerns, as well as visual aids and maps and spoken or sign language
interpreters to facilitate communication and discussion. The use of a CSM, for example, is
useful in public meetings to graphically reinforce the messages. It may be helpful to follow
up with meeting participants to inquire about the effectiveness of the meeting and whether it
met their needs. Other meeting follow-up activities could include responding to requests for
information, distributing meeting notes, and creating a mailing list.
Additional opportunities for the site planning team to communicate with the community in a
group setting include public availability sessions and public forums or poster sessions at
community group meetings or neighborhood board meetings. These options are a more
informal way of interacting with community members and they allow a casual "question and
answer" or discussion format as compared to the more formal presentation at a public
meeting.
Mass Media
The media can be the best means of reaching a large audience quickly. Extending
invitations to the media for important meetings, providing opportunities for media questions
250
Executive Order 13166, Improving Access to Services for Persons with Limited English Proficiency, directs federal
agencies to examine the services they provide, identify any need for services to those with LEP, and develop and
implement a system to provide those services so LEP persons can have meaningful access to them.
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to be addressed in a timely manner, and recognizing that the media control the content of
their publications all are important considerations when working with the media. The site
planning team can work with the Agency's regional site press officer to fostera relationship
with the media by sharing the Agency's rationale for its plans and actions. It is appropriate to
use the media to publicize a site-related decision, an upcoming meeting, changes in
schedule, or changes in activities or expectations. Press releases can be used to inform the
media of major site-related milestones.
Fact Sheets
Communities appreciate concise, easy-to-understand, and technically accurate fact sheets
on the history of the contamination, chemicals of concern, human health risk, planned
cleanup activities, and the vapor intrusion assessment and response actions. Be sure to
include who to contact for more information.
Because sites involving vapor intrusion can be complex, it may be useful to include
additional information in the fact sheets for home owners and renters, including information
about household products that may be potential sources of indoor air contamination, as well
as steps that can be taken to minimize these sources. EPA recommends preparing and
distributing periodicstatus updates and fact sheets to concerned community members
throughout the cleanup process.
Letters
Whenever there are plans to conduct indoor air sampling, EPA recommends sending a letter
to each building owner and renter explaining plans to conduct indoor air sampling and
requesting written permission for voluntary access to do so. In addition, a one-on-one
meeting with the building owner or renter is generally recommended to discuss sampling
efforts and access agreements in detail (see Section 9.3).
EPA also recommends that letters be sent to each building owner and renter to report
sampling results in a timely manner (see Section 9.4). These letters and meetings often are
part of a larger effort that also includes use of othercommunication strategies, such as
community meetings and in-person visits.
In-person Visits
EPA recommends individual, one-on-one communication with each property owner and
renter whenever possible.
Try to schedule in-person visits with individual property owners and renters. These visits
also may include owners and renters of properties located outside the planned
investigation area. The initial visit can be used to explain sampling plans in more detail,
answer questions, and obtain written permission to sample.
During the visit, the property owner or renter can be briefed about any instructions to
follow during sampling activities (for example, keep doors and windows closed during
sampling). A general survey of the building could be conducted to determine likely
sources of indoor air contaminants.
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EPA recommends the site planning team also describe to owners and renters the
sampling devices that will be used, what they look like, where they will be located, and
any restrictions or impediments to daily activities that may arise from the ongoing
sampling activities.
Information Repository
An information repository can be established and maintained prior to, during, and following
site activities, which is generally required for sites where remedial action or removal actions
(where on-site action is expected to exceed 120 days) are undertaken pursuant to CERCLA
The information repository is intended to include the administrative record, fact sheets,
question-and-answer sheets, and other site-related documents and be located reasonably
near the site. However, given the tremendous change in information technology, it may also
be appropriate to set up an Internet-based or digital repository (webpages) to share key
information. This depends on the community's ability to access and utilize this technology.
EPA recommends that community members be made aware of the information repository
through the other public outreach mechanisms described above (e.g., local media,
newsletters, and public meetings).
Electronic Notification
It also may be useful to establish a registration capability that allows interested community
members to sign up for automatic alerts to updates posted on the site website or email
listserv.
9.3 Addressing Building Access for Sampling and Mitigation
EPA recommends that all requests for access, as well as provision of access, be in writing in
order to document EPAs due diligence to protect human health at the site. EPA recommends
that the site planning team provide building owners and occupants with information about the
sampling device(s)) being used, including what they look like, where they will be located and
any restrictions or impediments to daily activities that may arise due to ongoing sampling.
In the case of an initial refusal to provide access, additional attempts for access are generally
recommended, although regional practices may vary. EPA recommends documenting all
attempts to gain access, for example using telephone conversation records, emails, or letters
sent to home or building owners.
Gaining access to owner-occupied residencesfor vapor intrusion sampling and mitigation may
be handled differently than for commercial buildings or rental properties.
Owner-Occupied Residences: Allowing EPA to sample or install mitigation systems in an owner-
occupied residence is a voluntary action. EPA generally encourages owners to take advantage
of an offer for an assessment and mitigation system, if necessary.
Rental Properties: Access may be voluntary or involuntary. Site planning teams often deal with
both owners and renters when there is a need to sample on, in, or under a rental property.
There are different legal and communication issues for owners and renters. Forexample, the
owner is responsible for granting access for sampling and for installation of mitigation
measures, if they are necessary; however, if the owner grants access, logistics normally are
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arranged with the renter. EPA recommends apprising both the owner and the renterof human
health risk that may be posed by vapor intrusion, which includes providing building-spedfic
sampling results to both parties when available. If the owner of a rental property refuses access,
EPA may, nevertheless, pursue access, in the interest of protecting the occupants, for
determining the need for response, choosing a response action, taking a response action, or
otherwise enforcing CERCLA or RCRA (EPA 1986,1987, 201 Oa). Notifying the owner of a
rental property of this statutory authority may help to avoid the need for legal action.
Nonresidential Buildings: Site managers also may need to sample on, in, or under
nonresidential buildings, such as schools, libraries, hospitals, hotels, and stores. In these
situations, broader outreach to the public may be appropriate in addition to maintaining direct
contact with the property owner. Similar to rental properties, access for sampling and for
implementation of mitigation methods, if they are necessary, may be voluntary or involuntary. If
the owner of a nonresidential building refuses access, EPA may, nevertheless, pursue access,
in the interest of protecting the occupants, for determining the need for response, choosing a
response action, taking a response action, or otherwise enforcing CERCLA or RCRA (EPA
1986, 1987,2010a).
Property Ownership Changes: For owners of homes or buildings who did not provide access for
assessment sampling or installation of a mitigation system, EPA recommends that the site
planning team make reasonable attempts to track ownership changes, although the appropriate
state, tribal, or local agency or PRP may be in a better position to track this information. For
example, reasonable attempts to make contact can be done by annually conducting drive-bys or
inspections and noting homes or buildings for sale, periodically checking on-line real estate
sales ortitle insurance listings, or using othermechanisms. Homes that were initially targeted
but not sampled can be reconsidered during the reviewer if there are major changes to the
toxicity values for the site contaminants of concern. Annually mailing notifications to buildings
not previously sampled is a means to foster reconsideration of testing with a change in
ownership. If ownership changes are noted, appropriate follow-up can be conducted with the
new home owner or building owner.
Federal statutory authority to access private property to conduct investigations, studies and
cleanups pursuantto CERCLA and RCRA is discussed in Section 1.2 of this Technical Guide.
9.4 Communication of Indoor Sampling Efforts and Results
The community involvement plan or public participation plan is intended to address community
concerns and participation regarding indoorair and sub-slab sampling. In addition to the general
community involvement activities occurring throughout the cleanup process (see Section 9.2),
the site planning team may choose to hold a community meeting to discuss indoor sampling
efforts and results. EPA recommends sending a letter to each home or building owner and
renter explaining plans to conduct sampling or providing sampling results. EPA recommends
that this letter be in addition to a one-on-one meeting with the building or home owner to discuss
access agreements, sampling efforts, and sampling results. Prompt communication of sampling
results to building or home owners is important as some people may choose to make
precautionary decisions prior to regulatory decisions on remediation or mitigation measures.
EPA recommends the site planning team inquire about stakeholder preferences for
confidentiality with regards to property-specific data. It may be appropriate to segregate data for
private residential properties versus community properties (e.g..schools, daycare centers,
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commercial buildings) or provide different types of property identifiers for these respective
building types in reports and maps and tables displayed at public meetings or otherwise made
available to the community.
Letters Transmitting Sampling Results
EPA recommends that the site planning team provide validated sampling results and
interpretations (e.g., chemicals of concern, associated risk assessment implications) in plain
English (and translations, if necessary) to property owners and renters in a timely manner (e.g.,
within approximately 30 days of receiving the results). EPA also recommends the transmittal
letter indicate what future actions (e.g., mitigation options), if any, are contemplated,252 based
on the sampling results, and contain additional site-specific and possibly building-specific
information, including, but not limited to:
Site and Home/Building Information.
o Site name and location of contamination.
o Date of sampling.
o Address of sampled home or building.
o Locations sampled (both indoor and outdoor).
Sampling Results
o Sampling results for site-related, vapor-forming chemical(s) and for any other chemicals,
if detected, including an explanation of results believed to be attributable to background
sources, if known.253
o Risk-based screening levels (for example, VISLs described in Section 6.5) or otherrisk-
based benchmarks used to explain and interpret the sampling results.
o Explanation and interpretation of sampling results, if known, which may include a
summary of the human health risk assessment, if available (see Section 7.4).254
Within the community of risk professionals, the phrase 'risk communication' has come to mean communication
that supplies lay people with the information they need to make informed independent judgments about human health
risk or public safety (Morgan et al. 1992). In this case about vapor intrusion, the ultimate goal of risk communication is
to assist stakeholders and the general public in understanding the investigation data and the rationale behind any
risk-informed decision, so they may arrive at a balanced judgement that reflects the factual evidence in relation to
their own interests and values.
252 This section may include an explanation of mitigation process and responsibilities and a timeline for further contact
regarding system installation and options. If a building mitigation system is recommended on the basis of a human
health risk assessment, EPA recommends that the site planning team explain that the risk calculation reflects
conservative, health-protective factors.
With such information, EPA can help advise citizens about the environmental and public health threats they face
that are within their control (e.g., from indoor sources). In cases where'background' contamination may pose a
human health risk, but its remediation is beyond the authority of the applicable statute, risk communication to the
public may be most effective when coordinated with public health agencies (EPA 2002e). The public may also be
advised about the scope and limits of EPA's statutory authorities.
Assessment uncertainty is generally an important factor in deciding how to act (Frewer2004); i.e., whetherto
reduce riskthrough response action or reduce uncertainty (e.g., through additional monitoring and data collection).
Rsk professionals, therefore, generally recommend that risk communication to stakeholders and the general public
characterize the sources of uncertainty, as well as the magnitude of uncertainty associated with a particular hazard
(see, for example, Frewer(2004) and Markon and Lemyre 92013)).
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o Simple tabulated and color-coded results (representing exceedances of human health
risk levels or no exceedance).
Diagrams/Illustrations
o Diagrams and illustrations of sampling devices.
o Diagrams and illustrations of sampling locations
o Diagrams of specific mitigation systems (e.g., howa SSD system works and looks).
Next Steps
o Actions that property owners and occupants can take to reduce vapor intrusion exposure
until mitigation systems are in place.
Information Sources
o Contact information for a person who can answer questions or supply further
explanations.
o The location of the site information repository or site website can be included as a
resource for public access to more detailed information and site documents.
9.5 Transmitting Messages Regarding Mitigation Systems
The initial notification to residents or building owners about mitigating vapor intrusion can be
delivered in various ways. A primary mechanism is a face-to-face meeting with the building
owner or occupant to explain the sampling results and discuss next steps, including installation
of a vapor intrusion mitigation system. EPA recommends that this meeting include a member of
the site planning team (RPM or OSC and risk assessor, for example), a representative from the
local health department or the Agency forToxic Substances and Disease Registry (ATSDR),
and the mitigation contractorscheduler. This meeting could discuss topics such as:
Sampling Results: Describe where samples were taken and the chemicals of concern, and
explain the results as related to site action levels. Any questions related to human health
risk can be answered by the risk assessor or public health representative at this time. For
questions or concerns regarding personal health, EPA recommends that residents and
building owners contact their medical professional.
Mitigation System Details: Describe the need for a mitigation contractor to visit the residence
to identify potential locations for the mitigation system. The property owner will need to be
present for the visit and will have input about where the system is installed, if they agree to
install such a system. Photos of a mitigation system (piping, system fan, number of holes
drilled in the slab, height of the vent on the outside of the residence, etc.) may be helpful.
EPA recommends that plans and schedules for periodic inspection, maintenance, and
monitoring also be described.
Access: EPA recommends advance planning to ensure building access to install, monitor
and maintain any mitigation system. Arrangements could be made at this meeting to sign an
additional access agreement for these activities, if needed.
Cost of the Mitigation System: Identify which party will pay for installation of the mitigation
system and anticipated property-owner costs. Forexample, EPA or a PRP may pay for the
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system installation, and the property owner or PRP may take responsibility to pay for the
monthly costs associated with the mitigation system.
Project Schedule and Next Steps: The meeting may be concluded by giving an overview of
the overall project timeline, including the appointment for the mitigation contractor visit and
system installation.
Notification also can be provided through the data transmittal letter. In many cases, however,
the decision to install mitigation systems will not have been made prior to the transmittal of
sampling results. In these situations, data transmittal letters can convey that EPA is reviewing
all data results for the affected area and considering appropriate next steps. Once the decision
document is signed, the site planning team can develop and mail a fact sheet to all community
members in the affected area, followed by a community meeting.
In addition, if a vapor intrusion mitigation system is installed, EPA recommends that the property
owner or renter be informed that the system normally is designed to protect the home or
building only against vapor-forming chemicals coming from the subsurface. A vapor intrusion
mitigation system generally will not protect the home against continuing indoor sources because
vapor intrusion mitigation systems typically are notindoorairfiltration systems.
EPA recommends that current owner-occupants be advised that if they decline or waive an offer
to install a vapor mitigation system, they might be responsible for the costs of installing and
maintaining their own system if they decide to do so at a later time. EPA also recommends
documenting any declination or waiver.
9.6 Addressing Community Involvement at Legacy Sites
Ongoing site activities with assessment components, such as remedial investigations and
monitoring, allow EPA to continually evaluate site conditions and adjust cleanup actions as
warranted. During periodic reviews or conducting other site activities, such as the FYR pursuant
to CERCLA, EPA has evaluated vapor intrusion where appropriate. In some instances, EPA has
newly identified vapor intrusion as an exposure pathway. These mature or "legacy" sites present
a unique challenge to site planning teams.
Conducting community involvement at legacy sites may be complicated by several factors
including:
A remedy for the control of exposure to volatile chemicals already has been installed,
proposed, or is under construction as part of the cleanup plan.
Ownership of properties previously exposed to VOCs has changed hands through
resale, foreclosure, or assumption of the property by second-generation homeowners.
These owners were not part of any original resolution of exposure issues and in many
cases may not be aware that a remediation or treatment was put in place.
Property owners and other community members who participated in prior cleanup efforts
may be reluctant to fully engage with efforts to reopen lines of investigation at their
properties.
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In these and similar circumstances, the challenge for Agency representatives is to resume
contact with communities who have put past difficulties behind them. In many cases, mailing
lists are outdated, previous reliable contacts no longer are available, and elected officials may
not have institutional memory of the events that prompted the remediation.
Strategies for Revitalizing Community Involvement at Legacy Sites
Every legacy re-entry will be a site-specific situation. Therefore, EPA recommends that events
and activities be planned to acknowledge and accommodate the inevitable changes in the
makeup of a community. In addition to the communication strategies and community
involvement techniques described in Sections 9.1 through 9.5, additional suggestions to ease
re-entry and revitalize community involvement at a legacy site include:
Reassess the community and the site by revisiting the site and the surrounding areas
and taking note of new construction.
Reintroduce yourself and the Agency to current municipal staff and check previously
used public venues for viability. Determine if new venues may be closer or more
accessible to the community.
If contacts within the community are still extant, reconnect; ask for updates on the
growth and stability of the community. If no viable contacts exist, attempt to cultivate new
ones.
Revise and update mailing lists and fact sheets.
As with all sites affected by vapor intrusion issues, be prepared to meet with property owners
door to door and to hold public meetings or forums to explain the current investigation and its
importance to protecting human health.
9.7 Property Value Concerns for Current and Prospective Property Owners
EPA recognizes that vapor intrusion impacts may have implications for property values. In some
instances, mitigation systems and other clean-up measures may help to restore property values.
Nevertheless, property value issues are outside the scope of Agency authority. In general, if
asked, EPA recommends that regional staff suggest that prospective buyers and sellers contact
real estate professionals and lenders from the local area with questions about property values. If
a home owner or renter has questions about vapor intrusion mitigation systems, EPA regions
can provide information that explains howvapor intrusion systems are designed to reduce
exposure to chemicals found in indoor air and to avert human health-related problems.
9.8 Additional Community Involvement Re sources
EPAs Superfund Community Involvement Program:
EPAs Superfund Community Involvement website contains many resources that may be helpful
for planning community involvement activities for other cleanup programs. This resource
includes a list of regional Superfund community involvement points of contact, a list of technical
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assistance and training resources, and descriptions and links to community involvement
policies, guidance and publications (see http://vwwv.epa.gov/superfund/communitv/).
EPA's Superfund Community Involvement Toolkit (Cl Toolkit):
While targeted to a Superfund Program audience, the Cl toolkit may be helpful to a wide variety
of users because it is a practical, easy-to-use aid for designing and enhancing community
involvement activities and contains tips on howto avoid some of the pitfalls common to the
community involvement process. The toolkit enables users to quickly review and adapt a variety
of community involvement tools to engage the community during all stages of the cleanup
process. Relevant tools include tips for conducting public availability and poster sessions and
public meetings, developing fact sheets, working with the media, planning communication
strategies, developing a Community Involvement Plan, and establishing an information
repository (see http://www.epa.gov/superfund/communitv/toolkit.htm).
EPA's Community Engagement Initiative:
The OSWER CEI is designed to enhance OSWER and regional offices' engagement with local
communities and stakeholders to help them participate meaningfully in government decisions on
land cleanup, emergency preparedness and response, and the management of hazardous
substances and waste (see http://www.epa.gov/oswer/engagementinitiative/).
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10.0 GLOSSARY
The following definitions are provided for purposes of this Technical Guide:
accumulate Increase gradually in amount as time passes. Note that
there will be a finite maximum amount, which will be
determined by site- and building-specific conditions and
will reflect a balance among physical processes (e.g.,
soil gas entry, air exchange).
active Involving mechanical operations; Compare with passive.
active depressurization technology Vapor intrusion mitigation method that creates a driving
force for airflow from the building into the subsurface by
lowering the pressure belowthe slab, thereby reducing
vapor intrusion (soil gas entry into a building).
acute Refers to repeated or single exposure for24 hours
duration or less. Compare with short-term and
subchronic.
advection As it pertains to soil gas, refers to bulk movement in the
vadose zone induced by spatial differences in soil gas
pressure. The direction of advective vapor transport is
always toward the direction of lower air pressure.
aerobic Describes a process or activity requiring oxygen.
Compare with anaerobic.
air exchange rate Rate of air infiltration into a building through windows,
doorways, intakes and exhausts, 'adventitious openings'
(e.g., cracks and seams that combine to form the
building envelope), plus natural and mechanical
ventilation.
ambient air The outdoor air surrounding a building or site.
anaerobic Describes a process or activity requiring the absence of
oxygen. Compare with aerobic.
analyte A substance for which identification (of presence) and/or
quantification (of amount, such as concentration) is/are
sought by instrumental measurement.
attenuation Decrease in vapor concentration in soil gas emanating
from a subsurface vapor source along the migration
route towards and into a building (indoor air)
attenuation factor The ratio of the indoor air concentration arising from
vapor intrusion to the soil gas concentration at the source
or a depth of interest in the vapor migration route.
background Refers to a vapor-forming chemical(s) or location(s) that
is(are) not influenced by the releases from a site, and is
usually described (EPA 1989,1995c, 2002e) as naturally
occurring or anthropogenic: 1) Anthropogenic- natural
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and human-made substances present in the environment
as a result of human activities and not specifically related
to the site-related release in question; and, 2) Naturally
occurring- substances present in the environment in
forms that have not been influenced by human activity.
Background may include a vapor-forming chemical(s)
present in indoor air due to human activities that is(are)
not related to vapor intrusion or site-related
contamination.
background vapor concentration This term may include the concentration of a vapor-
forming chemical in indoor air that fits within the definition
of "background" above. Information on background
concentrations of vapor-forming chemicals in indoor air
"is important to risk managers because generally EPA
does not clean up to concentrations below natural or
anthropogenic background levels" (EPA 2002e).255
background vapor source Theorigin(s) and location(s) of a vapor-forming
chemical(s), other than vapor intrusion, and not
associated with or emanating from a site-related
release(s) to the environment. Background vapor
sources may include indoor or outdoor sources. See
also indoor vapor source, outdoor vapor source, and
background vapor concentration; compare to subsurface
vapor source.
biodegradation Decomposition or breakdown of a substance through the
action of microorganisms (such as bacteria or fungi).
brownfield A parcel of real estate that is abandoned or inactive or
may not be operated at its fully beneficial use and on
which expansion or redevelopment is contemplated or
reasonably expected; distinguished from "greenfield"
because expansion or redevelopment may be
complicated by the presence of vapor-forming chemicals
in the subsurface environment.
building a structure that is intended for human occupancy and
use. This would include, for instance, homes, offices,
stores, commercial and industrial buildings, etc., but
would not normally include sheds, carports, pump
houses, or other structures that are not intended for
human occupancy.
building survey Refers generically to gathering--by observation,
interviews, reviewing documents and records or other
means information about existing buildings, including,
It should, however, be noted that some EPA regulations (e.g., indoor radon standards under 40 CFR 192.12) are
inclusive of background.
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but not limited to, location, use, occupancy, basic
construction (e.g., foundation type), heating ventilation
and cooling systems, potential indoor sources of vapor-
forming chemicals, and anticipated susceptibility to soil
gas entry (e.g., presence of radon mitigation system).
capillary fringe The porous material just above the ground water table
which may hold water by capillarity (a property of surface
tension that draws water upwards) in the smaller void
spaces.
chlorinated hydrocarbon (CMC) Compound comprised solely of the elements chlorine,
hydrogen and carbon. Includes dry-cleaning solvents
such as tetrachloroethylene (PCE) and degreasing
solvents such as trichloroethylene (TCE) and 1,1,1-
trichloroethane (TCA).
chronic Refers to repeated exposure for more than
approximately 10% of the life span (approximately seven
years) in humans. Compare with subchronic.
concentration Amount (mass) of a vapor-forming chemical contained in
a unit quantity (e.g., volume) of a specific medium (e.g.,
air, soil gas).
conceptual site model (CSM) Narrative description of the current understanding of the
site-specific conditions, which, in the case of vapor
intrusion, include the nature, location, and spatial extent
of the source(s) of vapor-forming chemicals in the
subsurface and the location, use, occupancy, and basic
construction of existing buildings. A CSM represents an
adaptation of a general conceptual model to account for
and reflect site- and/or building-specificconditions. See
also model.
crawl space A type of basement in which one cannot stand up the
height may be as little as one foot, and the bottom
surface is often bare soil.
complete (vapor intrusion)
pathway The vapor intrusion pathway is referred to as "complete"
for a building or collection of buildings when five
conditions are met under current conditions: (1) a
subsurface source of vapor-forming chemicals is present
underneath or near the building(s); (2) vapors form and
have a route along which to migrate (be transported)
toward the building(s); (3) the building(s) is (or are)
susceptible to soil gas entry, which means openings exist
for the vapors to enter the building(s) and driving forces
exist to draw the vapors from the subsurface into the
building(s); (4) one or more vapor-forming chemicals
comprising the subsurface vapor source(s) is (or are)
present in the indoor environment; and (5) the building(s)
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is (or are) occupied by one or more individuals when the
vapor-forming chemical(s) is (or are) present indoors.
data objectivity Refers to the accuracy, reliability, and absence of bias in
the information; scientific information will generally attain
this criterion when the original or supporting data are
generated using sound research, investigatory, or
statistical methods.
data quality objective (DQO) Performance and acceptance criteria that clarify study
objectives, define the appropriate type of data, and
specify tolerable levels of potential decision errors that
will be used as the basis for establishing the quality and
quantity of data needed to support decisions.
data utility Refers to the usefulness (e.g..relevance, importance) of
the information to reaching a conclusion or judgment
(e.g., is the vapor intrusion pathway complete or
incomplete? Does the vapor-forming chemical in indoor
air arise from background sources or vapor intrusion?
Does vapor intrusion pose an unacceptable human
health risk in a specific building?)
diffusion Random motion that affects the distribution of molecules
when there are spatial differences in chemical
concentrations in the fluid (e.g., soil gas, indoor air,
groundwater). The net direction of diffusive transport is
toward the direction of lower concentrations.
driving force refers to the combination of: (i) pressure differences
between a building interior and the subsurface or
ambient air, which foster vapor intrusion and infiltration,
respectively, via advection; and (ii) concentration
differences between a building interior and the
subsurface or ambient air, which foster vapor transport
via diffusion.
early action Refers to a response action undertaken early in the
cleanup process to achieve prompt risk reduction. Also
see response action and pre-emptive mitigation.
evidence A fact or other information (i.e., datum) ascertainable by
direct observation, interviews, review of records and
documents, instrumental analysis in a lab or field setting,
research and testing, sampling of environmental media
(e.g., indoor air, soil gas, groundwater), statistical
analysis, or other means, which is useful forforming a
conclusion or judgment; each distinguishable datum is
referred to as a line of evidence, which may be
categorized into scientific realms (e.g., geology, biology,
physics) or investigatory objectives (e.g., characterization
of subsurface vapor source, accounting for background
sources)
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exposure Opportunity to come into contact with vapor-forming
chemicals (via inhalation, in the case of vapor intrusion).
exposure assessment Process of characterizing the magnitude, frequency, and
duration of exposure to a vapor-forming chemical, along
with the characteristics of the population exposed.
exposure control Modification of a property or building intended to reduce
or eliminate human exposure to hazardous vapors in
buildings or explosive vapors in structures, which arise
from the vapor intrusion; such controls may include
engineered methods (e.g., activedepressurization
technologies, mechanical ventilation, indoor air
treatment) or non-engineered methods (e.g., institutional
controls, such as deed notices and land use restrictions)
exposure pathway The physical course a vapor-forming chemical takes from
its source (e.g., groundwater) to the individual (in a
building in the case of vapor intrusion).
exposure route The way in which a vapor-forming chemical enters a
human body (i.e., inhalation in the case of vapor
intrusion).
flux The rate of movement of mass through a unit cross-
sectional area per unit time in response to a
concentration gradient or a driving force for advection.
gas A fluid (as air) that has neither independent shape nor
volume but tends to expand indefinitely; a state of matter
in which the matter concerned occupies the whole of its
container irrespective of its quantity.
grab sample A sample of air collected over a short (practically
instantaneous) duration. Compare with time-integrated
sample.
hazard index (HI) The sum of hazard quotients for substances that affect
the same target organ or organ system. Because
different pollutants can cause similar adverse health
effects, it is often appropriate to combine hazard
quotients associated with different substances.
hazard quotient (HQ) The ratio of the potential exposure to the substance and
the level at which no adverse effects are expected. If the
HQ is calculated to be equal to or less than 1, then no
adverse health effects are expected as a result of
exposure. If the HQ is greater than 1, then adverse
health effects are possible.
hazardous Involving or exposing one to threat of adverse health
effects (due to toxicity) or loss of loss of life or welfare
(due to explosiveness).
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Henry's Law Coefficient or Constant... Ratio of a chemical's vapor pressure in air to its solubility
in water. Generally reported for standard reference
temperature, such as 25 ฐC.
human exposure pathway Away that people may come into contact with
environmental contaminants while performing their day-
to-day indoor activities.
human health risk assessment The evaluation of scientific information on the hazardous
properties of vapor-forming chemicals (hazard
assessment and characterization), the dose-response
relationship, and the extent of human exposure to those
agents. The product of the risk assessment is a
statement regarding the probability that populations or
individuals so exposed will be harmed and to what
degree and describing the principal technical
uncertainties (i.e., risk characterization).
hydrocarbon Compound comprised solely of the elements hydrogen
and carbon. See also chlorinated hydrocarbon and
petroleum hydrocarbon.
inclusion zone Land area within which EPA recommends assessing the
vapor intrusion pathway, which extends beyond the
aggregate boundaries of the site-specific source(s) of
vapor-forming chemicals.
indoor vapor source Refers to a vapor-forming chemical(s) in indoor air which
originates within a building. Indoor sources of vapor-
forming chemicals may include, but are not limited to,
use and storage of consumer or household products, use
or storage of industrial materials or products, combustion
processes, activities or operations within a building, and
releases from interior building materials (e.g., off-gases
from furniture or clothing); for example, operational use
or storage of chemicals in an industrial building may
represent an indoor vapor source separate from a site-
related release. Also see background vapor source;
compare to subsurface vapor source.
infiltration Air leakage into a building through random cracks,
interstices, and other unintentional openings in the
building envelope.
institutional control (1C) Non-engineering measures intended to affect human
activities in such a way as to prevent or reduce exposure
to hazardous substances. For example, ICs may be used
to restrict certain land uses, buildings, or activities that
could otherwise result in unacceptable exposure to the
vapor intrusion pathway. Generally, four categories of
ICs are recognized: governmental controls; proprietary
controls; enforcement tools; and informational devices.
They are almost always used in conjunction with, or as a
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supplement to, other cleanup measures such as
treatment or containment.
interim action Refers to a response action that is undertaken to protect
human health, but is limited in scope and objective (e.g.,
does not accomplish complete or final remediation of
subsurface vapor sources). Also see response action.
interzonal air flow Movement or transport of air through doorways,
ductwork, and service chaseways that interconnect
rooms or zones within a building.
lines of evidence Data collected and weighed together in supporting
assessments of the vapor intrusion pathway, which are
identified and described throughout Sections 2 through 7
inclusive. See also evidence.
lower explosive limit (LEL) The lowest concentration at which a gas or vapor is
flammable or explosive at ambient conditions.
mitigation Interim actions taken to reduce or eliminate human
exposure to vapor-forming chemicals in a specific
building arising from the vapor intrusion pathway;
compare with remediation.
model Refers to a description of a system. In the case of vapor
intrusion the 'system' will generally consist of a
subsurface source of vapors, one or more buildings
potentially subject to soil gas entry, and the soil
underlying the building(s). Conceptual models (i.e.,
models that are conceptual) are comprised of narrative
descriptions that identify the primary physical elements
and processes of the system and the interactions
between and relationships among them; for example,
Section 2 of this document provides a general
conceptual model of howvapor intrusion can arise and
why it may be variable overtime and in space. A
mathematical model is an expression of a conceptual
model, which uses mathematical symbols and language
to identify key elements (e.g., variables) and processes.
Generally, mathematical models are highly idealized or
simplified descriptions, compared to the complex
systems they represent. Forexample, Johnson and
Ettinger (1991) formulated an idealized mathematical
model of vapor intrusion. In the case of a physical model,
the description is provided using physical objects, which
may or may not have full functionality; for example, a
physical model of a construction project might differ from
a planned system in its scale, but would necessarily
show significant elements in relationship to each other.
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near-source Refers to a soil gas sample collected within a practically
short distance from subsurface vapor source
nonresidential building Refers to a building other than a home; includes, but is
not limited to, institutional buildings (e.g., schools,
libraries, hospitals, community centers and other
enclosed structures for gathering, gyms and other
enclosed structures for recreation); commercial buildings
(e.g., hotels, office buildings, many (but not all) day care
centers, and retail establishments); and industrial
buildings where vapor-forming chemicals may or may not
be routinely used or stored. Compare with residential
building; see also building.
outdoor vapor source Refers to a vapor-forming chemical(s) present in outdoor
(ambient) air. Sources of vapor-forming chemicals in
outdoor air may include, but are not limited to, releases
from industrial facilities, vehicle exhaust, yard
maintenance equipment, fuel storage tanks, paintor
pesticide applications, agricultural activities, and fires, as
well as site-related contamination, activities, and
operations (e.g., emissions from remediation equipment).
Also see background vapor source; compare to
subsurface vapor source.
passive Not involving mechanical operations; Compare with
active.
petroleum hydrocarbon (PHC) Hydrocarbons derived from petroleum and present in
various refined products of petroleum (such as
automotive gasoline, dieselfuel, lubricating oils). See
also hydrocarbon.
potentially complete (vapor intrusion ..
pathway) The vapor intrusion pathway is referred to as'potentially
complete' for a building when: a subsurface source of
vapor-forming chemicals is present underneath or near
an existing building or a building that is reasonably
expected to be constructed in the future; vapors can form
from this source(s) and have a route along which to
migrate (be transported) toward the building; and three
additional conditions are reasonably expected to all be
met in the future, which may not all be met currently (i.e.,
the building is susceptible to soil gas entry, which means
openings exist for the vapors to enter the building and
driving forces exist to draw the vapors from the
subsurface through the openings into the building; one or
more vapor-forming chemicals comprising the
subsurface vaporsource(s) is (or will be) present in the
indoor environment; andthe building is orwill be
occupied by one or more individuals when the vapor-
forming chemical(s) is (or are) present indoors.
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preemptive mitigation (PEM) Implementation of systems or control measures to
mitigate the vapor intrusion pathway as an early action,
even though all pertinent lines of evidence have not yet
been completely developed to characterize the vapor
intrusion pathway for the subject building(s). Also see
early action and response action.
preferential migration route Naturally occurring subsurface feature (e.g..gravel lens,
fractured rock) or anthropogenic (human-made)
subsurface conduit (e.g., utility corridoror vault,
subsurface drain) that is expected to exhibit little
resistance to vapor flow in the vadose zone (i.e., exhibits
a relatively high gas permeability) or groundwaterflow
(i.e., effectively exhibits a relatively high hydraulic
conductivity), depending upon its location and orientation
relative to the water table and ground surface, thereby
facilitating the migration of vapor-forming chemicals in
the subsurface and towards or into buildings
pressure difference/differential
pressure Difference between the air pressure within a building and
the subsurface environment or ambientair. Can promote
advective flow of gas into or out of a building through
pores, cracks, or openings in the building foundation or
envelope.
radon A radioactive gas formed during the radioactive decay of
radium, which occurs naturally in many geologic settings.
reasonable maximum exposure
(RME) A semi-quantitative term, referring to the lower portion of
the high end of the exposure distribution; conceptually,
above the 90th percentile exposure but less than the 98th
percentile exposure.
reasonable worst case A semi-quantitative term, referring to the upper portion of
the high end of the exposure distribution, but less than
the absolute maximum exposure.
reference concentration (RfC) An estimate of the continuous inhalation exposure to the
human population (including sensitive subgroups) that is
likely to be without appreciable risk of deleterious effects
during a lifetime.
remediation Refers to interim and final cleanups, whether conducted
pursuant to RCRA corrective action, the CERCLA
removal or remedial programs, or using EPA brownfield
grant funds with oversight by state and tribal response
programs. In addition to permanent remediesfor
subsurface vaporsources, site remediation may also
entail implementation of institutional controls and
construction and operation of engineered systems.
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residential building refers to a building used as or intended for use as a
home; includes, but is not limited to, single-family
detached homes with foundations, trailer homes, multi-
unit apartments and condominiums. Compare with non-
residential building; see also building.
response action any action taken to reduce or eliminate human exposure
to or risk posed by hazardous vapors in buildings and
structures, which arise from the vapor intrusion pathway;
these actions may include engineered exposure controls
in a specific building(s), non-engineered exposure
controls, remediation of subsurface vaporsources, and
associated monitoring to assess effectiveness and
protectiveness. Also see remediation and mitigation.
risk Probability of an adverse human health effect (due to
toxicity) or physical hazard (e.g., due to potential for
explosion) caused under specificcircumstances by a
vapor-forming chemical.
risk communication The process of exchanging information about health
threats and levels or significance of human health risk.
risk management The process of determining whether response action(s)
is(are) warranted to protect human health and, if so,
selecting response actions to implement.
screening Process of comparing concentrations of vapor-forming
chemicals in a specific medium (e.g., indoor air, soil gas,
crawl space air, groundwater)to screening levels to
identify sites, buildings, or chemicals unlikely to pose a
health concern through the vapor intrusion pathway
versus those warranting further investigation or analysis.
screening level Risk-based concentrations derived from standardized
equations combining exposure information and
assumptions with toxicity values.
short-term Refers to repeated exposure for more than 24 hours, up
to 30 days. Compare with acute and subchronic.
significant opening Refers to refer to an atypical form and amount of an
opening in a building (e.g., a sump, an unlined crawl
space, an earthen floor), which could facilitate greater
amounts of soil gas entry, all else being equal. Forms of
openings typically expected to be present in all buildings
include cracks, seams, interstices, and gaps in basement
floors and walls or foundations and perforations due to
utility conduits.
site The geographical area where investigation and
evaluation of the presence of vapor-forming chemicals is
desired; in many situations, it includes areas surrounding
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a facility where a release to the subsurface environment
is known or suspected to have originated.
soil gas The gas present underground in the pore spaces
between soil particles.
soil gas concentration Vapor concentration in a soil gas sample. Sub-slab soil
gas is found immediately beneath a building. Near-
source or exterior soil gas samples are collected at other
depths and typically outside the building footprint.
source strength Vapor concentration(s) of vapor-forming chemical(s)
arising from a subsurface vapor source.
stakeholder A person, group, community, or corporate entity with an
interest in activities at a site with subsurface
contamination.
subchronic Refers to repeated exposure for more than 30 days, up
to approximately 10% of the life span (approximately
seven years) in humans. Compare with short-term and
chronic.
subsurface remediation Response action that eliminates or substantially reduces
the level of vapor-forming chemicals in the subsurface
vapor source via treatment or physical removal.
Compare with mitigation.
subsurface vapor source Refers to a vapor-forming chemical(s) present in the
subsurface environment arising from a release(s) to the
environment. A subsurface vapor source may occur as a
non-aqueous-phase liquid (NAPL), adsorbed-phase
contamination, ordissolved-phase contamination, which
may be present in the vadose zone, in groundwater, or
within sewers and other conduits. Information on
subsurface vaporsources is important to risk managers
because response actions are generally warranted when
vapor intrusion is determined to pose unacceptable
human health risks. Compare with background vapor
source and background vapor concentration.
termination criteria Refers to numeric cleanup levels for each site-specific
contaminant and narrative cleanup objectives that are to
be attained by the response actions.
time-integrated sample Sample collected over an extended period of time to
account for temporal variations in vapor concentrations.
Compare with grab sample.
toxicity value Refers to an inhalation unit risk (IUR) for potential cancer
effects or an inhalation reference concentration (RfC)for
potential non-cancer effects of a vapor-forming chemical.
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vadose zone The soil zone between land surface and the groundwater
table within which the moisture content is less than
saturation (except in the capillary fringe). Soil pore
spaces not occupied by moisture contain (soil) gas. Also
referred to as the "unsaturated zone."
vapor A substance in the gaseous state as distinguished from
the liquid or solid state.
vapor intrusion The migration of potentially hazardous vapors from any
subsurface contaminant source, such as contaminated
soil or groundwater, through the vadose zone and into a
building or structure.
vapor source The place and form of origin of chemical vapors. Also
see background vapor source, indoor vapor source,
outdoor vapor source, and subsurface vapor source.
vapor-forming chemical A volatile chemical that EPA recommends be routinely
evaluated during a site-specific vapor intrusion
assessment, when it is present as a subsurface
contaminant.
volatile chemical Chemical with a vapor pressure greater than 1 milliliterof
mercury (mm Hg), or Henry's law constant greater than
10~5 atmosphere-meter cubed per mole.
volatility The tendency of a substance to form vapors, which are
molecules in a gaseous state, and escape from a liquid
or solid source. This tendency is directly related to a
substance's vapor pressure and Henry's law constant
and is indirectly related to a substance's molecular
weight (i.e., substances with lower molecular weights
tend to volatilize more readily than substances with
similar molecular structures that have higher molecular
weights).
watertable The water surface in an unconfined aquifer at which the
fluid pressure in the pore spaces is at soil gas pressure.
weight of evidence Refers to a conceptual approach to data evaluation, in
which each of several lines of evidence is critically
appraised for its quality (e.g., utility, objectivity) and
systematically assessed for its logical support for a
particular conclusion, as well as alternative conclusions;
the appraisals consider lines of evidence individually and
in light of other lines of reliable evidence for purposes of
determining whether a particular conclusion is supported
by the preponderance of the evidence and is consistent
with the conceptual site model; this 'weighing' concept
does not entail a quantitative (a priori) scheme to score
or rank the individual lines of evidence. See also
evidence.
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work plan A site-specific document that includes a project
description, projectobjective(s), historical information
about the site.
worst case A semi-quantitative term, referring to the absolute
maximum plausible exposure (i.e., a bounding- high-
impact- case).
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U.S. Environmental Protection Agency, Environmental Response Team (EPA-ERT).2001b.
Standard Operating Procedures, Soil Sampling (SOP2012). July 11. Currently available online
at: http://www.epaosc.ora/sites/2107/Files/2012-r10.pdf
U.S. Environmental Protection Agency, Environmental Response Team (EPA-ERT).2001c.
Standard Operating Procedures, Soil Gas Sampling (SOP2042). April 18. Currently available
online at: http://www.epaosc.ora/sites/2107/files/2082-rOO.pdf
U.S. Environmental Protection Agency, Environmental Response Team (EPA-ERT). 1995.
Standard Operating Procedures, Summa Canister Sampling (SOP 1701). July 27. Currently
available online at: http://www.epaosc.ora/sites/2107/files/1704-R01 .pdf
U.S. Environmental Protection Agency, Region 9 (EPA-Region 9). 2010. R9's
"RARE'Opportunityto Improve Vapor Intrusion Indoor Air Investigations. EPA Vapor Intrusion
Forum. June 14.
U.S. Environmental Protection Agency, Risk Assessment Forum (EPA-RAF). 2014. Framework
for Human Health Risk Assessment to Inform Decision Making. Office of Science Advisor,
Washington, D.C. April. Currently available online at:
http://www2.epa.aov/sites/production/files/2014-12/documents/hhra-framework-final-2014.pdf
213
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U.S. Environmental Protection Agency (EPA-SPC). 2006. Peer Review Handbook, 3rd Edition.
EPA/1 OO/B-06/002. Science Policy Council, Washington DC. Currently available online at:
http://vwwv2.epa.gov/sites/production/files/2014-
12/documents/peer review handbook 3rd ed.pdf
U.S. Postal Service (USPS). 2009. Vapor Intrusion Guidance, Volume 1. September.
Vroblesky, D.A., M.D. Petkewich, M.A. Lowery, and J.E. Landmeyer. 2011. Sewers as a source
and sink of chlorinated-solvent groundwater contamination, Marine Corps Recruit Depot, Parris
Island, South Carolina. Groundwater Monitoring & Remediation 31(4):63-69.
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APPENDIX A
RECOMMENDED SUBSURFACE-TO-INDOOR AIR ATTENUATION FACTORS
A.1 INTRODUCTION
This Technical Guide includes recommended medium-specific (groundwater, soil gas, and
indoor air) Vapor Intrusion Screening Levels (VISLs) that are intended to help identify those
sites unlikely to pose a health concern from vapor intrusion and identify areas or buildings that
may warrant further investigation of the vapor intrusion pathway. These VISLs are
recommended for use in evaluating the concentrations of vapor-forming chemicals measured in
groundwater, "near-source" exteriorsoil gas, and sub-slab soil gas in residential and
nonresidential settings where the potential for vapor intrusion isunderinvestigation.
The subsurface VISLs are developed considering a generic conceptual model for vapor
intrusion consisting of a groundwater or vadose zone source of vapor-forming chemicals that
diffuse upwards through unsaturated soils towards the surface and enter buildings. The
underlying assumption forthis generic model is that subsurface characteristics will tend to
reduce or attenuate soil gas concentrations as vapors migrate upward from the source and into
structures. Section 6.5.2 describes this conceptual model further. In general, EPA recommends
considering whether the assumptions underlying the generic conceptual model are attained at
each site. The Vapor Intrusion Screening Level (VISL) Calculator User's Guide (EPA2015a)
provides additional information about the technical basis for deriving the VISLs.
Comparison of sampling results to medium-specific VISLs (see Section 6.5.4) comprises one
line of evidence in the multiple-lines-of-evidence approach described in this Technical Guide
(see, for example, Sections 7.1 and 7.2). The subsurface (groundwater and soil gas) VISLs
(CVISL) are calculated using risk-based, screening levels for indoor air (C^et/a) and a medium-
specific, subsurface-to-indoor air attenuation factor (avi), as follows:
Equation A.1
otff
The risk-based, indoor air screening levels (Ctargetja) are calculated according to the guidance
provided in Risk Assessment Guidance for Superfund (RAGS) Pa/tF (EPA 2009) as
implemented in EPAs Regional Screening Levels (RSLs) forChemical Contaminants at
Superfund Sites (http://www.epa.gov/reg3hwmd/risk/human/rb-concentration table/). The
medium-specific, attenuation factors (ซw) recommended for calculating the subsurface VISLs
are derived from information in EPA's Vapor Intrusion Database: Evaluation and
Characterization of Attenuation Factors for Chlorinated Volatile Organic Compounds and
Residential Buildings (E PA 2 012 a).
This appendixdescribes the technical basis for the selection of the subsurface-to-indoorair
attenuation factors (ซF/) that are recommended for use in calculating the VISLs for groundwater,
sub-slab soil gas, "near-source" exteriorsoil gas, and crawl space air, according to Equation
A.1.
A-1
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June 201 5 Assessing and Mitigating the Vapor Intrusion Pathway from
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A.2 DEFINITION AND DESCRIPTION OF ATTENUATION FACTOR
Vapor attenuation refers to the reduction in concentration of vapor-forming chemicals that
occurs during vapor migration in the subsurface, coupled with the dilution that can occur when
the vapors enter a building and mixwith indoor air (Johnson and Ettinger 1991). The aggregate
effect of these physical and chemical attenuation mechanisms can be quantified through the
use of a subsurface-to-indoor air vapor intrusion attenuation factor (av,\ which is defined as the
ratio of the indoor air concentration arising from vapor intrusion (CM.W) to the subsurface vapor
concentration (C^) at the source or a depth of interest in the vapor migration route (EPA
2012a):
ฃMzI^ ฃMz!I Equation A.2
As defined here, the vapor attenuation factor is an inverse measurement of the overall dilution
that occurs as vapors migrate from a point of measurement in the subsurface into a building;
i.e., attenuation factor values decrease with increasing dilution of vapor concentration.
Subsurface vapor concentrations (CSv) may be measured directly under a building (often called
sub-slab soil gas or just sub-slab), measured exterior to a building at any depth in the
unsaturated zone (often called exterior soil gas), or derived from groundwater concentrations by
converting the dissolved concentration to a vapor concentration assuming equilibrium conditions
(i.e., by multiplying the groundwater concentration by the chemical's dimensionless Henry's law
constant for the groundwater temperature in situ) (EPA 2001); also see Appendix C of this
Technical Guide.
Subfloor vapor concentrations may also be measured in building crawl spaces. Although crawl
space samples are not strictly subsurface samples, they represent the vapor concentration
underlying a building's living space. Thus, crawl space samples may be evaluated in a manner
similar to subsurface vaporsamples.
A.3 RECOMMENDED ATTENUATION FACTORS
This section summarizes the technical basis and rationale for EPA's recommended attenuation
factors for groundwater, sub-slab soil gas, exterior soil gas, and crawl space air, as follows:
Section A.3.1 summarizes EPA's database of empirical attenuation factor values and the
results of analyzing that database.
Section A.3.2 identifies the recommended empirically based attenuation factors for
groundwater.
Section A.3.3 identifies the recommended attenuation factor for sub-slab soil gas and
presents a theoretical analysis that supports the selection of the recommended
empirically based value.
Section A.3.4 recommends a generic attenuation factor for exterior soil gas and
discusses its basis, justification, and limited applications.
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Section A.3.5 identifies the recommended attenuationfactorforcrawlspace vapor.
Section A.3.6 presents a reliability analysis of the recommended genericattenuation
factors.
A.3.1 ERA'S VAPOR INTRUSION DATABASE (EPA2012A)
The information in EPA's Vapor Intrusion Database: Evaluation and Characterization of
Attenuation Factors for Chlorinated Volatile Organic Compounds and Residential Buildings
(EPA 2012a) is used to derive recommended attenuation factor values for use in evaluating
subsurface sample concentrations collected as part of vapor intrusion investigations. EPA's
vapor intrusion database consists of numerous pairings of concentrations in indoor air and
subsurface samples (groundwater, sub-slab soil gas, exterior soil gas, and crawlspace vapor)
from actual sites. It represents the most comprehensive compilation of vapor intrusion data for
chlorinated hydrocarbons (CHCs) available at this time.
EPA's vapor intrusion database was analyzed and screened to reduce the impacts of
background sources to indoorair concentrations. The resulting data distributions are considered
representative of vapor intrusion of CHCs from subsurface vapor sources into residential
buildings for most conditions. These distributions serve as the basis for identifying the high-end
(conservative) attenuation factors for those media.
Table A-1 and Figure A-1 (Table 19 and Figure 34, respectively, in EPA (2012a)) present and
compare the distributions of the attenuation factors (groundwater, exterior soil gas, sub-slab soil
gas, and crawl space) that remain after applying the respective source strength and indoorair
screens considered most effective at redudng the influence of background contributions to
indoor air concentrations. These data demonstrate that the attenuation factor distributions
obtained for groundwater, sub-slab soil gas, and crawl spaces for multiple buildings and sites
are consistent with the conceptual model for vapor intrusion, which predicts that greater
attenuation is expected with greater depths to the vapor sources or vapor samples. As shown in
Table A-1 and Figure A-1, the paired groundwater-indoorair data generally exhibit greater
attenuation (lower attenuation factors) than the paired sub-slab soil gas-indoorair data, which
in turn exhibit greater attenuation than the paired crawl space-indoor air data.
A.3.2 RECOMMENDED ATTENUATION FACTORS FOR GROUNDWATER
To account for the inherenttemporal and spatial variability in indoorair and subsurface vapor
concentrations, the 95th percentile value of the "source-screened" groundwater data subset in
EPA 2012a is recommended as a reasonably conservative generic attenuation factor, after
considering a range of values. Thus, for groundwater, the recommended generic
attenuation factor (agw) is 0.001. This value is considered to apply for any soil type in the
vadose zone (excepting where preferential vapor pathways are present; see Section 5.4) in
cases where the groundwater is greater than five feet below the ground surface. If the depth to
groundwater is less than five feet belowthe building foundation, investigation of the indoor
space is recommended, as there is potential for contaminated groundwater to contact the
building foundation, eitherbecause the capillary fringe intersects the building foundation or
groundwater fluctuations results in groundwater wetting the foundation.
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Table A-2 (Table 13 in EPA (2012a)) provides statistics and Figure A-2 (Figure 28 in EPA
(2012a)) shows box-and-whisker plots for individual sites compared with the statistics for the
combined set of screened groundwater attenuation factors. This table and figure showthat the
95th percentile value of the combined groundwater-indoorair measurements is considered
appropriate forestimating reasonable maximum indoor air concentrations that might be
observed at a site due to vapor intrusion. The majority of sites and buildings would be expected
to exhibit lower indoor air concentrations.
A factor that commonly results in greater attenuation (lower attenuation factors) is the presence
of laterally extensive, unfractured fine-grained sediment in the vadose zone. Table A-3 (Table
14 in EPA (2012a)) provides selected statistics and Figure A-3 (Figure 29 in EPA (2012a))
shows the box-and-whisker plots for the groundwater attenuation factors for three soil types.
Comparing each descriptive statistic (except for the 25th percentile values) indicates that the
attenuation factorvalues for residences overlying soils classified as "very coarse" generally are
larger than those for residences overlying soils classified as "coarse," which are larger than
those for soils classified as "fine." This pattern is consistentwith the conceptual model for vapor
intrusion; smaller attenuation factors, which indicate greater reduction in vapor concentration,
would be expected in vadose zones with finer-grained soils, when all other factors (e.g., depth
to groundwater, biodegradability of the volatile chemicals) are the same. The 95th percentile
value of the coarse-grained soil is equal to the genericvalue, as expected, since coarse-grained
soil provide low resistance to vapor transport and thus would be expected to yield high-valued
attenuation factors. Where fine-grained sediments underlay buildings, however, more
attenuation is expected and observed in the database. Thus, a semi-site-specific attenuation
factor of 0.0005 maybe used at sites where laterally extensive fine-grained sediment has
been demonstrated through site-specific sampling to underlay buildings being
investigated for vapor intrusion.
A.3.3 RECOMMENDED GENERIC ATTENUATION FACTOR
FOR SUB-SLAB SOIL GAS
To account for the inherent temporal and spatial variability in indoor air and subsurface vapor
concentrations, the 95th percentile value of the "source-screened" sub-slab data subset in EPA
(2012a) is recommended as a reasonably conservative generic attenuation factor, after
considering a range of values. Thus, for sub-slab soil gas, the recommended generic
attenuation factor (ass) is 0.03.
The selection of this value can be supported by theoretical analysis. Specifically, a simple mass
balance analysis, assuming a well-mixed interior volume and steady-state conditions, indicates
that the theoretical (true) sub-slab soil gas attenuation factor can be expressed as the ratio of
the soil gas entry rate to the building ventilation rate (Song et al., 2011; EPA 2012a) for cases
where there is no background contribution to the indoorair concentration. Using median values
for residential building volume and air exchange rate (395 m3 and 0.45 ACH, respectively)
provided in the Exposure Factors Handbook 2011 Edition (EPA, 2011)and a mid-range value of
5 L/min for soil gas entry rate in sandy materials (EPA 2002, AppendixG), the central tendency
value of the sub-slab soil gas attenuation factor (according to Equation 4a therein), is expected
to be approximately 0.002. Using upper-end (1 Oth percentile) values for residential building
volume and air exchange rate (154 m3 and 0.18 ACH, respectively (EPA 2011)) and soil gas
entry rate (10 L/min), an upper-end value of 0.02 for the sub-slab soil gas attenuation factor is
obtained. These values agree well with the 95th percentile and 50th percentile (median) values
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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(0.03 and 0.003, respectively) obtained from the source-screened data. These calculations
buttress the conclusion that the sub-slab attenuation factordistributions summarized in EPA's
vapor intrusion database report can be considered representative of vapor intrusion of CHCs
into residential buildings for most conditions.
Table A-4 (Table 10 in EPA (2012a)) provides statistics and Figure A-4 (Figure 25 in EPA
(2012a)) shows box-and-whisker plots for individual sites compared with the statistics for the
combined set of screened sub-slab attenuation factors. This table and figure showthat the 95th
percentile value of the combined sub-slab-indoor air measurements is considered appropriate
for estimating reasonable maximum indoor air concentrations that might be observed at a site
due to vapor intrusion. The majority of sites and buildings would be expected to exhibit lower
indoor air concentrations.
A.3.4 RECOMMENDED ATTENUATION FACTOR FOR "NEAR-SOURCE"
EXTERIOR SOIL GAS
Based upon the conceptual model for vapor intrusion, the attenuation factors forexterior soil
gas data would be expected to be less than those for sub-slab soil gas, because the former
includes an additional contribution from attenuation through the vadose zone, and greaterthan
those for groundwater vapors for a given building at a site where groundwater is the primary
subsurface source of vapors. The distributions of exterior soil gas attenuation factors shown in
Table A-1 and Figure A-1 do not exhibit this expected relationship. In addition, a comparison of
exterior soil gas to sub-slab soil gas concentrations for buildings where both types of samples
were collected, shown in Figure A-5 (see Figure6 in EPA (2012a)), suggests that a substantial
proportion of the exterior soil gas data in the database, particularly shallow soil gas data, may
not be representative of soil gas concentrations directly underneath a building. On this basis,
shallow exterior soil gas sampling data generally are not recommended for purposes of
estimating indoor air concentrations and the exteriorsoil gas attenuation factors in Table A-1
are not recommended for use in deriving generic attenuation factors.
Based upon the data in Figure A-5, "deep" exterior soil gas data appearto more reliably reflect
sub-slab concentrations beneath buildings. On this basis, "near-source" soil gas sampling data
(i.e., collected in the vadose zone immediately above each vaporsource) generally are allowed
for purposes of assessing vapor concentrations that may be in contact with the building's sub-
slab, as discussed further in Section 6.4.4. However, the same conservative attenuation factor
value for sub-slab soil gas is recommended for use with "near-source" exterior soil gas data for
this purpose. Thus, for "near-source" exteriorsoil gas, the recommended generic
attenuation factor is 0.03.
A-5
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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A.3.5 RECOMMENDED ATTENUATION FACTOR FOR CRAWLSPACE VAPOR
The distribution of attenuation factors presented in Figure A-1 show that attenuation between
building crawlspaces and living spaces is limited. To account for the inherenttemporal and
spatial variability in indoor air and crawlspace vapor concentrations, the 95th percentile value of
the "indoor air-screened" crawlspace data subset in EPA (2012a) is recommended as a
reasonably conservative generic attenuation factor, after considering a range of values. Thus,
for crawl space vapor the recommended generic attenuation factor is 1.0 (0.9 rounded up
to 1.0).
A.3.6 RELIABILITY ANALYSIS OF THE RECOMMENDED SUBSURFACE-TO-
INDOORAIR GENERIC ATTENUATION FACTORS
An analysis was performed to determine the reliability of these recommended attenuation
factors for screening in residences in EPAs vapor intrusion data base with measured indoor air
concentrations exceeding target levels corresponding to a cancer risk of 10"6 and a hazard
quotient of 1. The reliability analysis was performed separately for each medium by determining
the number of correct assessments and the number of false negatives for a range of attenuation
factors. The potential incidence of false negatives is a critical criterion, because the primary
objective of risk-based screening is to identify sites or buildings unlikely to pose a health
concern through the vapor intrusion pathway (see Section 6.5.1).
For the purposes of this analysis:
A correct assessment is deemed to occur either (1) when a chemical's measured indoor
air concentration exceeds the target level and the measured subsurface vapor
concentration also exceeds the appropriate medium-specific VISL calculated using the
specified generic attenuation factor, or (2) when a chemical's measured indoor air
concentration is belowthe target level and the measured subsurface vapor
concentration also is belowthe appropriate medium-specific VISL calculated using the
recommended generic attenuation factor. Correct assessments in this analysis represent
a correct decision based on subsurface concentration data regarding the potential for
vapor intrusion to pose indoor air concentrations that exceed target risk-based
concentrations in affected buildings.
A false negative is deemed to occur when a chemical's measured indoor air
concentration exceeds the target level, but the measured subsurface vapor
concentration does not exceed the appropriate medium-specific VISL calculated using
the specified generic attenuation factor. False negatives in this analysis represent the
potential for making an incorrect decision based on subsurface concentration data
regarding the potential for vapor intrusion to pose indoor air concentrations that exceed
target risk-based concentrations in affected buildings.
This assessment uses the Data Consistency Subset of the EPAs vapor intrusion database for
residential buildings (i.e., before screening to minimize the impacts of background contributions
to indoor air as described in EPA (2012a)). This subset was chosen to allowfor the possibility
that background indoor air contributions were incorrectly identified and removed from further
analysis in the "source-screened" data subsets presented in EPA (2012a). Thus, false negatives
may appear if indoor or ambient (outdoor) sources of VOCs are present and they exceed the
A-6
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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indoor air target level. This choice of datasets provides a conservative estimate of the frequency
of false negatives identified by this reliability analysis. Even lower rates of false negatives would
be obtained when considering the "source-screened" data subsets, described in EPA (2012a),
in which the impacts of background contributions to indoor air are minimized.
The results of this assessment are shown in Figures A-6 through A-8 for sub-slab soil gas,
groundwater, and exterior soil gas.256 The essential results are as follows:
The recommended generic attenuation factors yield low rates of false negatives (< 2%)
for all three media when individual pairs of samples are evaluated together.
The recommended generic attenuation factors for groundwater, exterior soil gas, and
sub-slab soil gas provide generally high rates of correct assessments when individual
pairs of samples are evaluated together: 78% for groundwater; 76% for exterior soil gas;
and 87% for sub-slab soil gas. Higher rates of correct assessments are expected for
sub-slab soil gas than for the other subsurface media, likely due to the closer spatial
correspondence of building sub-slab soil gas and indoorair samples.
The rates of correct assessments appear to level off in Figure A-6 through A-8 at about
the point on the x-axis where the recommended generic attenuation factors occur.
The rates of false positives using the Data Consistency Subset can be inferred from
Figure A-6 through A-8. This analysis indicates that use of ground water data or exterior
soil gas data is more likely to incorrectly identify a site or building as warranting further
investigation than is use of sub-slab soil gas data.
Compared to the values estimated in Figures A-6 through A-8, significantly higher rates of a
correct assessment (and, hence, lower rates of false negatives and false positives) are
reasonably anticipated to be realized by following this Technical Guide. Specifically, collecting
multiple samples to characterize spatial and temporal variability (see, forexample, Section 6.4),
collecting multiple lines of additional evidence (see, for example, Section 6.3 and 7.1), and
weighing this information together (see, for example, Sections 6.3 and 7) are reasonably
expected to significantly reduce the "error rates" estimated in this reliability analysis, which are
based upon comparison of individual pairs of indoorair and subsurface sample concentrations.
As previously stated, this Technical Guide includes subsurface VISLs that are intended to help
identify those sites with the potential to pose a vapor intrusion concern. The reliability analysis
described above suggests the recommended attenuation factors, on which the recommended
VISLs are based, can reasonably be expected to provide an acceptably small probability of
'screening out' sites that pose a vapor intrusion concern and a high probability of correctly
identifying sites or buildings that may pose a vapor intrusion concern.
The reliability assessment was not conducted for craw I space data, because the distribution of attenuation factors
presented in Figure A-1 show that attenuation between building crawlspaces and living spaces is limited.
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
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A.4 CONSIDERATIONS FOR NONRESIDENTIAL BUILDINGS
The recommended attenuation factors (see Sections B.3.2 through B.3.5)are proposed for use
for nonresidential buildings as well as residential buildings. The rationale is that, in many
geographic locations, some commercial enterprises have been established in converted
residential buildings. Although used for commercial purposes, such buildings can reasonably be
expected to exhibit similar susceptibility to vapor intrusion and similar interior mixing and dilution
(and, hence, similar attenuation factors) as residential buildings represented in EPA's vapor
intrusion database. In addition, McDonald and Wertz (2007) found that sub-slab attenuation
factors for commercial and institutional buildings in Endicott, New York, which were not
"extraordinarily large", were not substantially different than those for residential buildings in the
same area.
There are theoretical considerations to support expectations that larger nonresidential buildings
that are constructed on thick slabs will have lower attenuation factors than residential buildings.
These considerations include:
Given that the size (e.g., interior height and footprint area) and air exchange rate tend to
be larger for many nonresidential buildings (see, for example, Table A-5), it is expected
that building ventilation rates for many nonresidential buildings would be higher than
those for residential buildings. A higher ventilation rate is expected to result in greater
overall vapor dilution as vapors migrate from a subsurface vaporsource into a building.
On this basis, many nonresidential buildings would be expected to have lower
attenuation factors than those for residential buildings, all else being equal.
Comparing buildings with slab-on-grade construction, nonresidential buildings tend to
have thicker slabs than residential buildings. With thicker slabs, a given amount of
differential settling would be expected to lead to less cracking in the slab and would be
less likely to create cracks that extend across the entire slab thickness. Buildings with
thicker slabs would, therefore, be expected to exhibit lower soil gas entry rates, all else
being equal.
Where appropriate, EPA may consider appropriate building-specific data, information, and
analysis when evaluating vapor intrusion into large nonresidential buildings.
A.5 CITATIONS (APPENDIX A)
Johnson, P.C.,andR.A. Ettinger. 1991. Heuristic model for predicting the intrusion rate of
contaminant vapors into buildings. Environmental Science & Technology 25:1445-1452.
McDonald, G.J. and W.E. Wertz. 2007. PCE, TCE, and TCA vapors in subslab soil gas and
indoor air: A case study in upstate New York. Groundwater Monitoring & Remediation 27(4):86-
92.
Song, S., F. C. Ramacciotti, B. A. Schnorr, M. Bock, and C.M. Stubbs. 2011. Evaluation of
EPA's empirical attenuation factordatabase. EM: The Magazine for Environmental Managers
February: 16-21. Currently available online at http://pubs.awma.Org/gsearch/em/2011/2/song.pdf
U.S. Environmental Protection Agency (EPA). 2015a. Vapor Intrusion Screening Level (VISL)
Calculator, User's Guide. Currently available online at:
A-8
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
http://vwwv.epa.gov/oswer/vapohntrusion/guidance.html
U.S. Environmental Protection Agency (EPA). 2012a. U.S. EPA's Vapor Intrusion Database:
Evaluation and Characterization of Attenuation Factors for Chlorinated Volatile Organic
Compounds and Residential Buildings. EPA 530-R-10-002. Currently available online at
http://www.epa.gov/oswer/vaporintrusion/documents/OSWER 2010 Database Report 03-16-
2012 Final.odf
U.S. Environmental Protection Agency (EPA). 2012b. Conceptual Model Scenarios for the
Vapor Intrusion Pathway. EPA-530-R-10-003. Currently available online at
http://www.epa.gov/oswer/vaporintrusion/documents/vi-cms-v11final-2-24-2012.pdf
U.S. Environmental Protection Agency (EPA). 2011. Exposure Factors Handbook- 2011
Edition. Office of Research and Development, Washington, D.C. EPA/600/R-090/052. Currently
available online at http://www.epa.gov/ncea/efh/pdfs/efh-complete.pdf
U.S. Environmental Protection Agency (EPA). 2009. Risk Assessment Guidance for Superfund
(RAGS), Volume I: Human Health Evaluation Manual (Part F, Supplemental Guidance for
Inhalation Risk Assessment). Office of Superfund Remediation and Technology Innovation,
Washington, D.C. EPA-540-R-070-002. Currently available online at
http://www.epa.gov/oswer/riskassessment/ragsf/indexhtm
U.S. Environmental Protection Agency (EPA). 2002. OSWER Draft Guidance for Evaluating the
Vapor Intrusion to Indoor Air Pathway from Groundwaterand Soils (Subsurface Vapor Intrusion
Guidance). Office of Solid Waste and Emergency Response, Washington, D.C. EPA-530-D-02-
004. November. Currently available online at:
http://www.epa.gov/osw/hazard/correctiveaction/eis/vapor.htm
U.S. Environmental Protection Agency (EPA). 2001. Fact Sheet, Correcting the Henry's Law
Constant for Soil Temperature. Office of Solid Waste and Emergency Response, Washington,
D.C. Currently available online at
http://www.epa.gov/oswer/riskassessment/airmodel/pdf/factsheet.pdf
A-9
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TABLE A-1.
EXTERIOR SOIL GAS, SUB-SLAB SOIL GAS, AND CRAWL SPACE VAPOR AFTER APPLICATION OF THE
DATABASE SCREENS CONSIDERED MOST EFFECTIVE AT MINIMIZING THE INFLUENCE OF BACKGROUND
SOURCES ON INDOOR AIR CONCENTRATIONS.
Statistic
Min
5%
25%
50%
75%
95%
Max
Mean
StdDev
95UCL
Count All
Count >RL
Count 1,OOOXBkgd)
1 .OE-07
3.6E-06
2.3E-05
7.4E-05
2.0E-04
1 .2E-03
2.1E-02
2.8E-04
1 .OE-03
3.4E-04
774
743
31
24
Exterior Soil Gas
(SG > SOX Bkgd)
5.0E-06
7.6E-05
6.0E-04
3.8E-03
2.7E-02
2.5E-01
1.3E+00
5.0E-02
1.7E-01
7.8E-02
106
106
0
11
Sub-slab Soil Gas
(SS > SOX Bkgd)
2.5E-05
3.2E-04
1 .5E-03
2.7E-03
6.8E-03
2.6E-02
9.4E-01
9.2E-03
5.0E-02
1 .3E-02
431
411
20
12
Crawl Space
(IA > Bkgd)
5.7E-02
1.0E-01
2.2E-01
3.9E-01
6.9E-01
9.0E-01
9.2E-01
4.6E-01
2.8E-01
5.3E-01
41
41
0
4
Note: The applied database screensare groundwater (vapor) concentrations> 1 ,OOOX "background," exterior soil gas> SOX "background," sub-slab soil gas> SOX "background,"and
forcrawl space, indoorairconcentrations> 1X "background."SOURCE: Table 19in EPA (2012a).
-------
l.E+01
l.E+00
l.E-01
l.E-02
.0 l.E-04
TO
= l.E-05
l.E-06
l.E-07
Max
4 95th %
75th %
50th %
25th %
5th %
Min
I!
3 >-l
0 A
IS g
is
8 9
ซ J>
*!
i- O
O ifi
'>- A
S IS
ฃ J2.
O
W X
^^
Tn A
^ t/l
9
_g>
wl CQ
Figure A-1. Box-and-whisker plots summarizing attenuation factor distributions for groundwater, exterior soil gas, sub-slab soil gas, and crawl space
vapor after application of the database screens considered most effective at minimizing the influence of background sources on indoor air
concentrations. SOURCE: Figure 34 in EPA (2012a).
-------
TABLE A-2.
DESCRIPTIVE STATISTICS SUMMARIZING GROUNDWATER ATTENUATION FACTOR DISTRIBUTIONS FOR
INDIVIDUAL SITES COMPARED WITH THE COMBINED DATASET AFTER SOURCE STRENGTH SCREEN
(GROUNDWATER VAPOR CONCENTRATIONS > 1,000 TIMES "BACKGROUND").
Statistic
Min
5%
25%
50%
75%
95%
Max
Mean
StdDev
95UCL
Count All
Count >RL
Count
1.000X
Bkgd
1.0E-07
3.6E-06
2.3E-05
7.4E-05
2.0E-04
1.2E-03
2.1E-02
2.8E-04
1.0E-03
3.4E-04
774
743
31
|. |
<ฃ <ฃ
9.1E-06 2.5E-06
3.7E-06
1.4E-05 1.1E-03
1.1E-04
3.4E-04
2.8E-04
2 12
1 5
1 7
LiJ
O
Q.
CO
1.0E-06
1.1E-05
2.1E-05
3.9E-05
8.9E-05
6.8E-04
8.0E-04
1.2E-04
2.1E-04
1.9E-04
25
25
0
o
1.8E-06
3.4E-06
9.9E-06
2.2E-05
1.5E-04
5.4E-04
5.4E-04
1.1E-04
1.7E-04
1.8E-04
17
17
0
ฃ
CO
if, O
'> 3
CO CO
O LiJ
4.7E-05 3.6E-06
2.5E-04
4.3E-04 1.9E-03
2.4E-04 7.7E-04
8.1E-04
1.4E-03
2 6
2 6
0 0
.u
o
c
LiJ
1.9E-05
2.8E-05
2.8E-05
1.7E-04
7.0E-04
1.4E-03
1.5E-03
4.3E-04
4.8E-04
5.7E-04
32
22
10
CO
O
1.0E-07
9.7E-07
2.7E-06
1.2E-05
8.7E-05
2.9E-04
2.9E-04
7.5E-05
1.1E-04
1.2E-04
14
14
0
amilton-Sundstrand
^
9.6E-06
1.2E-05
5.8E-05
1.0E-04
1.5E-04
2.9E-04
5.2E-04
1.2E-04
9.8E-05
1.5E-04
32
32
0
arcros/Tri State
^
1.2E-06
2.5E-04
3.7E-03
7.1E-04
1.3E-03
1.7E-03
7
7
0
opewell Precision
^
2.5E-05
1.7E-04
2.9E-04
5.6E-04
1.2E-03
7.7E-03
7.7E-03
1.2E-03
1.8E-03
2.0E-03
17
17
0
s
| S
1 1
2.9E-06
4.0E-06
1.7E-05
4.7E-04 3.4E-05
1.4E-04
6.8E-04
2.3E-03
1.6E-04
3.6E-04
2.2E-04
1 93
1 93
0 0
ง Q. Q.
S LU LU
g Q Q
I ^ ^
8.6E-07 1.6E-04
2.9E-06
1.9E-05
8.8E-05 4.0E-05
2.7E-04
1.3E-03
2.4E-03 1.0E-03
2.6E-04 6.0E-04
4.5E-04
3.5E-04
63 2 1
63 2 1
000
ฃ
"o
^
1.3E-06
4.0E-06
1.9E-05
7.9E-06
9.3E-06
2.4E-05
3
3
0
ountainView
^
4.8E-07
3.3E-06
3.3E-05
9.7E-06
1.4E-05
2.3E-05
5
5
0
b
CO
o;
9.9E-06
3.1E-05
4.0E-05
2.7E-05
1.6E-05
5.4E-05
3
3
0
o;
1.7E-06
7.6E-06
2.8E-05
7.3E-05
1.5E-04
4.8E-04
1.8E-03
1.3E-04
1.9E-04
1.5E-04
329
329
0
CM - Cortlandville
00
5.9E-05
5.9E-05
5.9E-05
3.1E-04
1.7E-03
4.2E-03
6.6E-03
1.1E-03
1.6E-03
1.6E-03
28
21
7
ncasville
z>
3.3E-05
3.5E-04
4.8E-04
6.5E-04
1.8E-03
6.0E-04
5.1E-04
9.2E-04
9
9
0
CO
S
1.4E-06
1.7E-05
2.9E-05
8.2E-05
3.2E-04
1.4E-03
1.1E-02
4.9E-04
1.7E-03
9.2E-04
43
43
0
est Side Corp.
s
2.1E-06
1.3E-05
1.5E-05
3.7E-05
2.7E-04
4.3E-03
2.1E-02
1.1E-03
4.0E-03
2.3E-03
28
22
6
SOURCE: Table 13 in EPA (2012a).
-------
i Min
Figure A-2. Box -and-whisker plots summarizing groundwater attenuation factor distributions for individual sites compared with the
combined data set after Source Strength Screen (groundwater vapor concentrations > 1,000 times "background"). SOURCE:
Figure 28 in EPA (2012a).
-------
TABLE A-3.
DESCRIPTIVE STATISTICS SUMMARIZING GROUNDWATER ATTENUATION FACTOR DISTRIBUTIONS FOR
SPECIFIC SOIL TYPES AFTER SOURCE STRENGTH SCREEN.
Statistic
Min
5%
25%
50%
75%
95%
Max
Mean
StdDev
95UCL
Count All
Count >RL
Count
-------
ฃ l.OE-01 -
B
c l.OE-02 -
'i_
c 10E-03 -
01
S
| l.OE-04 -
a
i
0 l.OE-05 -
3
0
ซ l.OE-06 -
i nf-m -
<
=
ป
^
^
4
=
3
^^^^^^^^
^
C
^
S
r
L
< Max
> 95th %
-i 75th %
J 25th %
> 5th %
Fine
Coarse
V.Coarse
Figure A-3.Box-and-whisker plots summarizing ground water attenuation factor distributions for specific soil types after Source
Strength Screen. SOURCE: Figure 29in EPA(2012s).
-------
TABLE A-4.
DESCRIPTIVE STATISTICS SUMMARIZING SUB-SLAB ATTENUATION FACTOR DISTRIBUTIONS FOR INDIVIDUAL
SITES COMPARED WITH THE COMBINED DATASET AFTER SOURCE STRENGTH SCREEN (SUB-SLAB SOIL
GAS CONCENTRATIONS > 50 TIMES "BACKGROUND").
Statistic
SS > SOX
Bkgd
Min 2.5E-05
5%
3.2E-04
BillingsPCE
2.5E-05
9.6E-05
25% 1.5E-03 4.6E-04
50%
75%
2.7E-03 7.0E-04
6.8E-03 1.5E-03
95% 2.6E-02 2.6E-03
Max 9.4E-01 2.7E-03
Mean 9.2E-03 9.5E-04
StdDev 5.0E-02 7.7E-04
95UCL 1.3E-02 1.2E-03
No. of AFs 431 27
No. of AFs > RI-
ND, of AFs < RL
411
20
27
0
DenverPCEBB
1.1E-03
6.4E-03
4.1E-02
1.7E-02
1.9E-02
3.5E-02
5
5
0
Endicott
2.6E-04
6.9E-04
1.7E-03
2.6E-03
5.0E-03
1.1E-02
9.4E-01
8.5E-03
6.5E-02
1.6E-02
207
188
19
Georgetown
1.3E-03
1.9E-03
2.9E-03
2.0E-03
8.4E-04
3.5E-03
3
3
0
CJ
M
y,
ra
3.8E-04
4.5E-04
2.7E-03
l.OE-03
1.1E-03
2.3E-03
4
4
0
c
'3)
Hopewell Prec
Jackson
CO
LL.
1.5E-03 3.5E-05
1.9E-03 1.4E-04
5.0E-03 4.1E-04
l.OE-02 8.4E-03 1.9E-03
1.8E-02 5.3E-03
3.4E-02 3.2E-02
3.4E-02 4.2E-02
1.3E-02 8.4E-03 5.0E-03
l.OE-02 9.0E-03
1.7E-02 7.1E-03
19 1 52
19
0
1
0
52
0
^
ro
Q.
.O
6
5.0E-04
1.8E-03
2.8E-03
8.8E-03
3.3E-02
7.6E-03
1.1E-02
1.4E-02
9
9
0
Raymark
2.5E-04
1.2E-03
2.0E-03
5.5E-03
8.3E-03
2.1E-02
7.9E-02
7.4E-03
l.OE-02
9.2E-03
83
83
0
o
ro
= ฃ
1 1
1 !
^ i/>
<ฃ 01
u >
Ul >
3.4E-03 2.0E-04
3.6E-03
7.1E-03 5.9E-04
1.8E-02 1.5E-03
4.1E-02 9.7E-03
1.5E-01
1.5E-01 3.5E-01
4.1E-02 4.3E-02
5.0E-02 1.2E-01
6.8E-02 1.2E-01
12 9
12 8
0 | 1
SOURCE: Table 10 in EPA (2012a).
-------
June 2015
Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
o
0 1.0E-01
re
Li.
C
O
re
ง 1.0E-02
0)
^f
(/)
re
^ 1.0E-03
O
V)
&
& 1.0E-04
CO
1 OF n^
4
-
t
!
.1
Q
>
C
u
/
u
0
J
3)
:
3
<
3
V
3
)
^
U
^
c
i
j
C
t
=
J
)
)
J)
5
D
D
LJ
\
i
i
(
C
3
3
J
)
}
>
i
j
c
|
T
C
U
y .
T
v^ I ! s
= T ' ' 1
"*" < *
L -|-
<
\ 1 1 1 1 i 1 1 1 1
2ai~oo-'c>.cS
S> g ฃ! I ซ | &
Sg5: 6^5
0 2 ai w 0 ฐ
E & ^ a.
RT 50 times "background"). SOURCE: Figure 25 in EPA(20i2a).
-------
June 2015
Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
TABLE A-5
COMPARISON OF SIZE CHARACTERISTICS FOR RESIDENTIAL AND SOME
COMMERCIAL BUILDINGS
Building Parameter and
Units
ACHB|dg(1/hr), 1 0th percentile
HBidg (feet)
Value and Source for
Residential Building
0.18(EPA2011, Table 19-1)
8-feet ceiling height (EPA
201 1 , assumed value)
Value and Source for
Commercial Buildings,
Othe r Than Warehouses and
Enclosed Malls
0.6 (EPA 2011, Table 19-27)
1 2-feet ceiling height (EPA
201 1 , assumed value)
Subslab Concentrations {|ig/m3)
i nnn
i no
n 1
o o y
O
,te
.V* o
A X'ป
o A ''' o
" *1 " . *
0'' -A ^ 0
.'1$ fo0<> J 0"
' ป A ^S '-'*-'
' * A v a |
A A Jk A
^
s
s
s
s
0.1
10 100 1000 10000
Soil Gas Concentrations {|ig/mj)
Close and Deep SG
For Jiid Deep SG
A Far and Shallow SG
O Distance/Depth Unknown
One to One
100000
Figure A-5. Exterior soil gas versus sub-slab soil gas concentrations for buildings with both types of data in
EPA's vapor intrusion database differentiated qualitatively by horizontal distance to building
and depth to the exterior soil gas sample. SOURCE: Figure 6 in EPA(2012a).
A-18
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June 2015
Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
Subslab Soil Gas - Indoor
Reliability Analysis
100%
re
M
.0
ฃ 50%
u
1_
Ol
Q. 40%
30%
0.1 0.01
Subslab Soil Gas - Indoor Air Attenuation Factor
0.001
Figure A-6.Reliability Predictionsfor Alternative Choices of the Sub-slab Attenuation Factor
Based on a Comparison of Paired Data in the Data Consistency Screen Dataset
[tabulated allies shown below]
Reliability Analysis: Subslab Soil Gas -Indoor Air
Classification
Correct
FN
Total
SSAF = 1
551
0
767
SSAF = 0.1
630
7
767
SSAF = 0.03
669
16
767
SSAF = 0.02
674
21
767
SSAF = 0.01
689
26
767
SSAF = 0.002
683
56
767
SSAF = 0.001
689
68
767
SSAF
Correct
FN
1
72%
0%
0.1
82%
1%
0.03
87%
2%
0.02
88%
3%
0.01
90%
3%
0.002
89%
7%
0.001
90%
9%
A-19
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June 2015
Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
Groundwater- Indoor Air
Reliability Analysis
100%
90%
80%
O
E
QJ
Q.
0.1 0.01 0.001 0.0001
Groundwater - Indoor Air Attenuation Factor
0.00001
Figure A-7.Reliability Predictionsfor Alternative Choices of the Groundwater Attenuation Factor
Based on a Comparison of Paired Data in the Data Consistency Screen Dataset
[tabulated allies shown below]
Reliability Analysis: Groundwater -Indoor Air
Classification
Correct
FN
Total
GWAF = 1
240
0
810
GWAF = 0.01
319
2
810
GW AF = 0.002
442
4
810
GWAF = 0.001
635
6
810
GWAF = 0.0002
681
25
810
GWAF = 0.0001
703
54
810
GWAF = 0.00001
634
169
810
GWAF
Correct
FN
1
30%
0.0%
0.01
39%
0.2%
0.002
55%
0.5%
0.001
78%
0.7%
0.0002
84%
3.1%
0.0001
87%
6.7%
0.00001
78%
20.9%
A-20
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June 2015
Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
Exterior Soil Gas - Indoor Air
Reliability Analysis
80%
70%
C 40%
01
u
i_
Ol
Q.
30%
20%
10%
0.01
Exterior Soil Gas - Indoor Air Attenuation Factor
Figure A-8. Reliability Predictionsfor Alternative Choices of the Exterior Soil Gas Attenuation
Factor Based on a Comparison of Paired Data in the Data Consistency Screen Dataset
[tabulated \^lues shown below]
Reliability Analysis: Exterior Soil Gas -Indoor Air
Classification
Correct
FN
Total
SG AF = 1
88
0
176
SG AF = 0.6
90
0
176
SG AF = 0.3
102
0
176
SG AF = 0.1
126
1
176
SG AF = 0.03
133
4
176
SG AF = 0.02
132
6
176
SG AF = 0.01
132
15
176
SGAF
Correct
FN
1
50%
0%
0.6
51%
0%
0.3
58%
0%
0.1
72%
1%
0.03
76%
2%
0.02
75%
3%
0.01
75%
9%
A-21
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
APPENDIX B
DATA QUALITY ASSURANCE CONSIDERATIONS
B.1 INTRODUCTION
Site-specific investigations of the vapor intrusion pathway will generally entail the collection and
evaluation of environmental data and possibly the use of modeling. As noted in Exhibit B-1, EPA
generally recommends the use of a quality assurance project plan (QAPP) for the collection of
primary (and existing or secondary) data. A QAPP is a tool for project managers and planners to
document the type and quality of data needed to make environmental decisions and to describe
the methods for collecting and assessing the quality and integrity of those data. A QAPP is a
plan or roadmap intended to help a projectteam document howthey plan, implement, and
evaluate a project. It applies the systematic planning process and the graded approach for
collecting environmental data for a specific intended use. EPA standards governing the
collection of data are outlined in Exhibit B-1.
Exhibit B-1. EPA Data Standards
CIO 2105 (formerly EPA Order 5360; Policy and Program Requirements for the Agency-wide
Quality System, May 2000) is intended to promote the organization collecting or using the data
to (1) establish a Quality System and prepare and approve a QAPP for each project.
For clarity, CIO 2105 will be replaced by the following two standards:
CIO 2106-S-01 is the Quality Standard for Environmental Data Collection, Production,
and Use by EPA Organizations, also called "Internal Standard" (EPA2013a); and
CIO 2106-S-02 is the Quality Standard for Environmental Data Collection, Production,
and Use byNon-EPA (External) Organizations, also called "External Standard" (EPA
2013b).
These standards conform to EPA Quality Policy, CIO 2106.0, "Quality Policy"(EPA2008a),
Procedure for Quality Policy, CIO 2106-P-01.0, "Quality Procedure" (EPA2008b), and the
American National Standards Institute (ANSI) consensus standard, Quality Systems for
Environmental Data and Technology Programs-Requirements with Guidance for Use
(ANSI/ASQ 2004).
Two guidance documents accompany these standards:
EPA Guidance on Quality Management Plans (EPA2012b, CIO 2106-G02-QMP),
documents the quality system of the organization conducting environmental data
collection or using the data for EPA.
EPA Guidance on Quality Assurance Project Plans (EPA 2012a, CIO 2106-G-05)
focuses on projects requiring the collection of new data, projects using existing data, and
projects involving modeling.
B-1
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
EPA also encourages the use of the Uniform Federal Policy for Quality Assurance Project Plans
(UFP-QAPP) (EPA/DoD 2005) as a collaborative approach to fulfill the purposes of a QAPP,
especially for Federal Fadlities. OSWER Directive 9272.0-17, Implementation of the Uniform
Federal Policy for Quality Assurance Project Plans (UFP-QAPP) at Federal Facility Hazardous
Waste Sites(EPA2005a) and OSWER Directive 9272.0-20(EPA2005b) state thatQAPPs
prepared and approved under the UFP conform to EPAs quality standards and are consistent
with EPA Standards CIO 2106-S-O and CIO 2106-S-02, EPA's Quality Policy (EPA2008a), and
ANSI/ASQ 2004.
B.2 RECOMMENDATIONS
This appendix provides two recommendations concerning the key components of QAPP
development. These recommendations are not exhaustive, but are included as a starting point
as considerations before studying or applying EPA or UFP QAPP guidance.
Recommendation 1: Using the conceptual site model (CSM), develop the project plan and
QAPP through a process that involves all key players and share these materials with interested
parties in draft form so that potential study weaknesses can be addressed early. The CSM is
developed to portray the current understanding of site conditions, the nature and extent of
contamination, routes of contaminant transport, potential contaminant pathways, and potentially
exposed human population. Developing the CSM is the first step in EPAs DQO process.
Recommendation 2: Use systematic planning in developing project documents, including the
QAPP. Systematic planning is a science-based, common-sense approach designed to ensure
that the level of documentation and rigorof effort in planning is commensurate with the intended
use of the information and available resources. DQOs are a key component of systematic
planning and play a central role in the systematic planning process. DQOs generally are
addressed within the QAPP and typically are a critical element in the planning for environmental
investigations. Guidance on Systematic Planning Using the Data Quality Objectives Process
(QA/G-4) (EPA 2006) provides guidance addressing implementation of DQOs and application of
systematic planning to generate performance and acceptance criteria for collecting
environmental data.
Table B-1 summarizes the steps in the DQO process, the purpose of each step, and provides
some examples of how plans could be structured.
B-2
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June 2015
Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
TABLE B-1. EXAMPLE OF STEPS IN THE DQO PROCESS
DQO Step
Purpose of the DQO Step
Example Application for Vapor Intrusion
1. State the
Problem
Summarize the problem (e.g.,
the monitoring hypothesis, the
investigation objective(s)) for
which new environmental data
will be collected or modeling or
analysis will be performed.
Indoor air in one or more buildings overlying a
shallow plume of PCE-contaminatedgroundwater
is (are) to be sampled to determine whether PCE
is present. The original PCE release occurred at
an industrial site approximately 1,000 feet away
from the closest building.
2. Identify the
Decision
Identify the decision that will be
supported by the new data,
modeling or analysis.
The data will be used to support decisions about
whether additional indoor air sampling or
preemptive vapor intrusion mitigation will be
pursued in one or more buildings.
3. Identify the
Inputs to the
Decision
Identify the information needed
to support the decision,
including data gaps that warrant
collection of new information.
Indoor air sampling data for one or more
buildings, in conjunction with information about
measured or interpolated concentrations in
groundwater near or underneath the building(s).
4. Define the
Boundaries of
the Study
Specify the spatial and temporal
aspects of the environmental
media or endpoints that the data
are to represent to support the
decision.
The boundaries of this initial study area extend a
prescribed distance outside the lateral extent of
the plume. Eventually, the boundaries of a vapor
intrusion impact zone will be defined by the extent
to which indoor air contamination can be
associated with site-related contamination.
5. Develop a
Decision Rule
Develop a logical "if...then"
statement that defines the
conditions that will inform the
decision-maker to choose
among alternative decisions.
Buildings with detectable concentrations of PCE
in indoor air samples will be considered for
additional indoor air sampling or preemptive
vapor intrusion mitigation.
6. Specify
Tolerable Limits
on Decision
Errors
Specify acceptable limits on
decision errors, which are used
to establish performance goals
for limiting uncertainty in the
analysis.
EPA recommends analytical limits of detection be
less than risk-based screening levels for PCE to
ensure that a building's indoor air concentration is
not misidentified.
7. Optimize the
Design for
Obtaining Data
Identify the most resource-
effective sampling and analysis
design for generating the
information needed to satisfy the
DQOs.
Time-integrated samples will be collected in
basements and in the first above-ground level of
each building. The sampling and analysis plan
and approach will be documented in a QAPP.
B-3
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
B.3 CITATIONS AND REFERENCES (APPENDIX B)
ANSI/ASQ. 2004. Quality Systems for Environmental Data and Technology Programs -
Requirements with Guidance for Use. E4-2004. Currently available for purchase online at
http://webstore.ansi.org/FindStandards.aspx?SearchString=ansi+e4&SearchOption=0&PageNu
m=0&SearchTermsArrav=null|ansi+e4|null
U.S. Environmental Protection Agency (EPA). 2000. Policy and Program Requirements forthe
Agency-wide Quality System. CIO 2105. May. Currently available online at
http://www.epa.gov/irmpoli8/policies/21050.pdf
U.S. Environmental Protection Agency (EPA). 2005a. Implementation of the Uniform Federal
Policy for Quality Assurance Project Plans (UFP-QAPP) at Federal Facility Hazardous Waste
Sites. OSWER Directive 9272.0-17. June?. Currently available online at
http://www.epa.gov/fedfac/pdf/oswer qapp directive.pdf
U.S. Environmental Protection Agency (EPA). 2005b. Applicability of the Uniform Federal Policy
for Quality Assurance project Plans. EPA-505-04-900A, OSWER Directive 9272.0-20.
December 5. Currently available online at http://www.epa.gov/fedfac/pdf/oswer_9272.0^2Q.pdf
U.S. Environmental Protection Agency (EPA). 2006. Guidance on Systematic Planning Using
the Data Quality Objectives Process (QA/G-4). EPA-240-B-06-001. February. Currently
available online at http://www.epa.gov/qualitv/qs-docs/g4-final.pdf
U.S. Environmental Protection Agency (EPA). 2008a. U.S. Environmental Protection Agency
Quality Policy. CIO 2106.0. Currently available online at
http://www.epa.gov/irmpoli8/policies/21060.pdf
U.S. Environmental Protection Agency (EPA). 2008b. U.S. Environmental Protection Agency
Procedure for Quality Policy. CIO 2106-P-01.0. Currently available online at
http://www.epa.gov/irmpoli8/policies/21060.pdf
U.S. Environmental Protection Agency (EPA). 2012a. U.S. Environmental Protection Agency
Guidance on Quality Assurance Project Plans. CIO 2106-G-05 QAPP. January 17.
U.S. Environmental Protection Agency (EPA), 2012b. EPA Draft Final Guidance on Quality
Management Plans. CIO 2106-G02-QMP. January 17. Currently available online at
http://www.epa.gov/oeitribalcoordination/2106-G-05%20QAPPฐ/o20Final%20Draft%2001 -17-
12.pdf
U.S. Environmental Protection Agency (EPA). 2013a. Quality Standard for Environmental Data
Collection, Production, and Use by EPA Organizations. CIO2106-S-01.
B-4
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
U.S. Environmental Protection Agency (EPA). 2013b. Draft Final Quality Standard for
Environmental Data Collection, Production, and Use byNon-EPA (External) Organizations.
"External Standard." CIO 2106-S-02. February 22. Currently available online at
http://www.regulations.gov/#!documentDetail:D=EPA-HQ-OEI-2012-077 4-0002
U.S. Environmental Protection Agency and U.S. Department of Defense (EPA/DoD). 2005.
Uniform Federal Policy for Quality Assurance Project Plans (UFP-QAPP). Part 1: UFP-QAPP
Manual. EPA-505-B-04-900A, DTIC ADA 427785. March. Currently available online at
http://vwwv2.epa.gov/fedfac/uniform-federal-policv-qualitv-assurance-project-plans-evaluating-
assessing-and-documenting
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APPENDIX C
CALCULATING VAPOR SOURCE CONCENTRATION FROM
GROUNDWATER SAMPLING DATA
Correcting the Henry's Law Constant for Ground water Temperature
In the case of groundwater as the vapor source, the subsurface source concentration
(Csv) is estimated assuming that the vapor and aqueous phases are in local equilibrium
according to Henry's law such that:
csv = H'TS x cw Equation C. 1
where:
Csv = vapor concentration at the source of contamination (g/cm3-v),
H'TS = Henry's law constant at the system (groundwater) temperature
(dimensionless), and
Cw = concentration of volatile chemical in groundwater (g/cm3-w).
The Henry's law constants generally are reported fora temperature of 25 degrees Celsius (ฐC).
Table C-1 provides these values for the chlorinated hydrocarbons (CHCs) in the vapor intrusion
database. Average groundwater temperatures, however, are typically less than 25ฐC. In such
cases, use of the Henry's law constant at 25ฐC may over-predict the volatility of the contaminant
in water.
As described in EPA's So/7 Screening Guidance (EPA 1996), the dimensionless form of
the Henry's law constant at the average groundwater temperature (H'gw) may be estimated
using the Clapeyron equation:
exp
1 1
T
*-
RxTg* Equation C.2
where:
AHv,gw= enthalpy of vaporization of the specific chemical at the groundwater
temperature (cal/mol),
Tgw = groundwater temperature (ฐK= ฐC + 273.15),
TR = reference temperature for the Henry's law constant (298. 1 5ฐK),
Rc = gas constant (= 1 .9872 cal/mol-ฐK),
HR = Henry's law constant for the specific substance at the reference temperature
(atm-m3/mol), and
R = gas constant (= 8.205 E-05 atm-m3/mol-ฐK).
C-1
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The enthalpy of vaporization at the groundwater temperature can be approximated from the
enthalpy of vaporization at the normal boiling point, as follows:
Equation C.3
where:
= enthalpy of vaporization at the groundwater temperature
(cal/mol),
= enthalpy of vaporization at the normal boiling point
(cal/mol),
Tc = critical temperature for specific chemical (ฐK),
TB = normal boiling point for specific chemical (ฐK),
H = exponent (unitless), and
all other symbols are as defined previously. Table C-1 provides the chemical-specific property
values used for temperature corrections to the Henry's law constant. Table C-2 provides the
value of n as a function of the ratio TB/TC. If site-specific data are not readily available for the
groundwater temperature, then Figure 1 of the EPA fact sheet, Correcting the Henry's Law
Constant for Soil Temperature (EPA 2001) can be used to generate an estimate.
Citations (Appendix C)
U.S. Environmental Protection Agency (EPA). 1996. Soil Screening Guidance: Technical
Background Document. Office of Solid Waste and Emergency Response, Washington, D.C.
EPA-540-R-95-128. Currently available online at:
http://www.epa.gov/superfund/health/conmedia/soil/introtbd.htm
U.S. Environmental Protection Agency (EPA). 2001. Fact Sheet, Correcting the Henry's Law
Constant for Soil Temperature. Office of Solid Waste and Emergency Response, Washington,
D.C. Currently available online at
http://www.epa.gov/oswer/riskassessment/airmodel/pdf/factsheet.pdf
U.S. Environmental Protection Agency (EPA). Regions 3, 6, and 9. 2011. Regional Screening
Levels for Chemical Contaminants at Superfund Sites. Region 3, Philadelphia, PA. November.
Currently available online at http://www.epa.gov/reg3hwmd/risk/human/rb-
concentration table/index htm
C-2
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June 2015 Assessing and Mitigating the Vapor Intrusion Pathway from
Subsurface Vapor Sources to Indoor Air
Lide, D.R. (Ed.). 1998. CRC Handbook of Chemistry and Physics, 79thEd. Boca Raton, FL.
CRC Press.
Mallard, W.G. and P.J. Linstrom (Eds.). 1998. NISTChemistry WebBook:NISTStandard
Reference Database Number 69. Gaithersburg, MD. National Institute of Standards and
Technology. November. Currently available online at http://webbook.nist.gov/chemistrv/
C-3
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Table C-1. Chemical-Specific Parameters for Adjusting Henry's Law Coefficients for Groundwater Temperature
Chemical
Abstracts
Service
Registry
Number
(CASRIM)
56-23-5
75-00-3
67-66-3
75-34-3
75-35-4
156-59-2
156-60-5
75-09-2
127-18-4
76-13-1
71-55-6
79-01-6
75-01-4
Alphabetized List of
Compounds
Carbon tetrachloride
Chloroethane (ethyl chloride)
Chloroform
Qchloroethane,1,1-
Qchloroethene, 1,1-
Qchloroethene,cis-1 ,2-
Qchloroethene,trans-1,2-
Methylene chloride
Tetrachloroethylene
Trichloro-1,2,2-
trifluoroethane, 1,1,2-
Trichloroethane, 1,1,1-
Trichloroethylene
Vinyl chloride (chloroethylene)
Henry's Law Constant
@25ฐC
HR
(atm-m /mol)
2.76E-02
1.11E-02
3.67E-03
5.62E-03
2.61 E-02
4.08E-03
4.08E-03
3.25E-03
1.77E-02
5.26E-01
1.72E-02
9.85E-03
2.78E-02
source
a
a
a
a
a
a
a
a
a
a
a
a
a
Henry's Law
Constant
@25ฐCg
H'R
(unitless)
1.13E+00
4.54E-01
1.50E-01
2.30E-01
1.07E+00
1.67E-01
1.67E-01
1.33E-01
7.23E-01
2.15E+01
7.03E-01
4.03E-01
1.14E+00
Normal Boiling Point
Tb
(ฐK)
3.50E+02
2.85E+02
3.34E+02
3.30E+02
3.05E+02
3.28E+02
3.28E+02
3.13E+02
3.94E+02
3.21 E+02
3.47E+02
3.60E+02
2.60E+02
source
b
b
b
b
b
b
b
b
b
b
b
b
b
Critical Temperature
Tc
(ฐK)
5.57E+02
4.60E+02
5.36E+02
5.23E+02
5.76E+02
5.44E+02
5.17E+02
5.10E+02
6.20E+02
4.87E+02
5.45E+02
5.44E+02
4.32E+02
source
h
f
h
h
h
h
h
h
h
f
h
h
h
Enthalpy of
vaporization at the
normal boiling point
AHv,b
(cal/mol)
7.13E+03
5.88E+03
6.99E+03
6.90E+03
6.25E+03
7.19E+03
6.72E+03
6.71 E+03
8.29E+03
6.46E+03
7.14E+03
7.51 E+03
5.25E+03
source
h
f
h
h
h
h
h
h
h
f
h
h
h
Sourcesand Footnotes
a Based on valuesreported in the U.S. EPA Regional Screening Tables. November2011 .Available online at: http://www.epa. gov/reg3hwmd/risk'human/rb-
concentration_table/Generic_Tables'xls'params_sl_table_run_NOV201 l.xls
b Experimental values. E PA 2009. Estimati on Programs Interface Suite forMicrosoftฎ Windows, V4.00.U.S EPA, Washington, DC, USA. Available online at:
http://www.epa.gov/opptintr/exposure/pubs/episuite.htm
f CRC HandbookofCherhstryandPhysics, 76th Edition
h EPA (2001). FACT SHEET Correcting the Henry's Law ConstantforSoil Temperature. Attachment.
g National Institute of Standardsand Technology (NIST). Chemistry WebBook. Available online at http://Webbooknist.gov/chemistry/
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Assessing and Mitigating the Vapor Intrusion Pathway from
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Table C-2. Values of Exponent n as a Function of TB/TC
Chemical-specific ratio TB/TC
<0.57
0.57 - 0.71
>0.71
^^^H H ^^^H
0.30
0.74
(Te/Tc) -0.116
0.41
C-5
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